The anatomy of Reddit: An overview of academic research <s> Introduction 2 The Reddit dataset <s> We analyze the structure and evolution of discussion cascades in four popular websites: Slashdot, Barrapunto, Meneame and Wikipedia. Despite the big heterogeneities between these sites, a preferential attachment (PA) model with bias to the root can capture the temporal evolution of the observed trees and many of their statistical properties, namely, probability distributions of the branching factors (degrees), subtree sizes and certain correlations. The parameters of the model are learned efficiently using a novel maximum likelihood estimation scheme for PA and provide a figurative interpretation about the communication habits and the resulting discussion cascades on the four different websites. <s> BIB001 </s> The anatomy of Reddit: An overview of academic research <s> Introduction 2 The Reddit dataset <s> Frigyes Karinthy, in his 1929 short story "Lancszemek" (in English, "Chains") suggested that any two persons are distanced by at most six friendship links.1 Stanley Milgram in his famous experiments challenged people to route postcards to a fixed recipient by passing them only through direct acquaintances. Milgram found that the average number of intermediaries on the path of the postcards lay between 4:4 and 5:7, depending on the sample of people chosen. We report the results of the first world-scale social-network graph-distance computations, using the entire Facebook network of active users (≈ 721 million users, ≈ 69 billion friendship links). The average distance we observe is 4:74, corresponding to 3:74 intermediaries or "degrees of separation", prompting the title of this paper. More generally, we study the distance distribution of Facebook and of some interesting geographic subgraphs, looking also at their evolution over time. The networks we are able to explore are almost two orders of magnitude larger than those analysed in the previous literature. We report detailed statistical metadata showing that our measurements (which rely on probabilistic algorithms) are very accurate. <s> BIB002 </s> The anatomy of Reddit: An overview of academic research <s> Introduction 2 The Reddit dataset <s> In the past few years, Reddit -- a community-driven platform for submitting, commenting and rating links and text posts -- has grown exponentially, from a small community of users into one of the largest online communities on the Web. To the best of our knowledge, this work represents the most comprehensive longitudinal study of Reddit's evolution to date, studying both (i) how user submissions have evolved over time and (ii) how the community's allocation of attention and its perception of submissions have changed over 5 years based on an analysis of almost 60 million submissions. Our work reveals an ever-increasing diversification of topics accompanied by a simultaneous concentration towards a few selected domains both in terms of posted submissions as well as perception and attention. By and large, our investigations suggest that Reddit has transformed itself from a dedicated gateway to the Web to an increasingly self-referential community that focuses on and reinforces its own user-generated image- and textual content over external sources. <s> BIB003 </s> The anatomy of Reddit: An overview of academic research <s> Introduction 2 The Reddit dataset <s> Online discussion in form of written comments is a core component of many social media platforms. It has attracted increasing attention from academia, mainly because theories from social sciences can be explored at an unprecedented scale. This interest has led to the development of statistical models which are able to characterize the dynamics of threaded online conversations. In this paper, we review research on statistical modeling of online discussions, in particular, we describe current generative models of the structure and growth of discussion threads. These are parametrized network formation models that are able to generate synthetic discussion threads that reproduce certain features of the real discussions present in different online platforms. We aim to provide a clear overview of the state of the art and to motivate future work in this relevant research field. <s> BIB004 </s> The anatomy of Reddit: An overview of academic research <s> Introduction 2 The Reddit dataset <s> Comunicacio presentada a la Eleventh International AAAI Conference on Web and Social Media, celebrada els dies 15 a 18 de maig de 2017 a Montreal, Canada. <s> BIB005 </s> The anatomy of Reddit: An overview of academic research <s> Introduction 2 The Reddit dataset <s> As researchers use computational methods to study complex social behaviors at scale, the validity of this computational social science depends on the integrity of the data. On July 2, 2015, Jason Baumgartner published a dataset advertised to include “every publicly available Reddit comment” which was quickly shared on Bittorrent and the Internet Archive. This data quickly became the basis of many academic papers on topics including machine learning, social behavior, politics, breaking news, and hate speech. We have discovered substantial gaps and limitations in this dataset which may contribute to bias in the findings of that research. In this paper, we document the dataset, substantial missing observations in the dataset, and the risks to research validity from those gaps. In summary, we identify strong risks to research that considers user histories or network analysis, moderate risks to research that compares counts of participation, and lesser risk to machine learning research that avoids making representative claims about behavior and participation on Reddit. <s> BIB006
Reddit (launched in 2005) is a social news aggregation, web content rating, and discussion website, ranked as #6 most visited website in the world with 234 million unique users (as of February 2018) BIB004 . A schematic structure of Reddit is illustrated in Figure 1 . Registered users submit posts that contain a title, an external link or a self-written piece of content, which immediately become available to the whole audience of Reddit for voting and commenting. The voting system permits only registered users to upvote (give a positive +1 vote) or to downvote (give a negative -1 vote) on posts and comments. Comments form a discussion tree, which can be Fig. 1 The schematic structure of the Reddit platform. The entry point is the top page of Reddit, which is fed of posts from the subreddits followed by a registered user (or from all subreddits, for an anonymous user), and ranked according to votes and posts' age. The user may further proceed to top page of a specific subreddit, where the feed narrows down to only posts from a chosen subreddit. Each post can be upvoted or downvoted and has an attached section of comments. The comments are structured as a rooted tree by the reply-to relation to other comments or the post itself. described as a rooted tree, where the root is a designated node representing the post itself and each other node represents a comment. There is a link between two nodes if there is a 'reply-to' relation between them. The huge posting space of Reddit is divided into subreddits -self-created communities of users, united by a certain topic. Every submitted post has subreddit name as an intangible attribute. Each subreddit and Reddit itself has a so-called "top page" -the feed where post titles with voting and commenting links are delivered to users. Two factors influence the post's ranking position there: 1) time and 2) voting score, or otherwise called karma, which is basically the difference between upvotes and downvotes. High score posts have a higher chance of appearing at the top page. However with time, newer information replaces the older in the feed. Users can follow subreddits, but not other users, which constitutes the main distinction with social network platforms, like Facebook or Twitter, where users follow a person and not a content. Other platforms have a similar structure. For example, Slashdot (launched in 1997) is made of news stories, together with comments moderated by selected users, but not by an open voting system 2 . Only a fixed number of topic-based subsections is available. Hacker News (launched in 2007) is an online community very similar to Reddit but with only two pre-made subreddits BIB005 . Digg (launched in 2004) acts currently as a news aggregator, but it formerly was a socially curated platform with a post submission, commenting and voting system like Reddit BIB002 . Meneame is a Spanish analogue of Digg, Barrapunto is also a Spanish version of Slashdot BIB001 . Etc. Fig. 2 The evolution of Reddit from Jan 2008 till Jan 2018: a) monthly counts of posts and comments, b) distribution of discussion sizes. One may notice an exponential increase in the activity counts, but the discussion size distribution follows a similar shape, close to the power-law with exponent α ranging from around 1.7 for early years to 1.9 for later. Reddit has gained a central place in the scientific literature thanks to the openness, richness and quality of its data, which allows to perform longitudinal studies of the whole system and, critically, to ensure reproducibility of the results. Jason Baumgartner, under the Reddit name Stuck In The Matrix, did a tremendous amount of work when attempted to collect a full dataset of posts and comments, going back to the creation of the site  . The figures of this Chapter have all been prepared from this dataset. For instance, basic numbers on the growth of the site and the total sizes of discussions are found in Figure 2 . His data repository also contains the data from the platform Hacker News  . Despite its recognized quality, the dataset is not flawless either. Gaffney and Matias BIB006 report several inconsistencies in the data. For example, comment and post data before 2008 appears to be hugely corrupted, having around 80% of posts missing, as well as 90% posts information of one month data at the interface between 2009 and 2010. In total, across the time interval Jan 2006 and Feb 2016, the authors report 0.043% missing comments and 0.65% missing posts. The risks of missampled data are obvious, but in large scale studies they may be safely disregarded due to their smallness. The system has sustained exponential growth BIB003 , thus data volume in early years is negligibly small compared to today's numbers. The newly published rescraped data must be free from the errors found in the data before 2008  . Although missing data causes reasonable inconsistensies, reported comments from authors, whose accounts were deleted, constitute around 25% of the data according to the letter appendix to BIB006 which imposes a greater obstacle on user-centric studies. In such case, the name of the author turns to default name " [deleted] ", but all the other information on the submitted post or comment remains in the system.