Differences between SSRN, arXiv, academia.edu, NBER working papers, et al Posting papers on depositories for working papers may lead to higher visibility than posting on a personal website. The depositories send out updates about new papers and people browse uploaded papers.  

That being said, what are the relative benefits of the different depositories? Some of them are more discipline-specific, but I work broadly in social science and any of them could work for researchers like me.
As a general rule, you should write in some medium if you read that medium. So you should send your work to a depository if you follow that depository. (See this advice on cross-listing in arXiv.)
If you do not follow any, start following them. If you follow several, choose the one that you prefer as a reader.