Is social media helpful for research dissemination? Is there any quantitative evidence that using social media helps academics disseminate their research results?  What about the cost-effectiveness of social media?

My guess is that search engines are much more important than social media, and take less effort?
An article from 2014 "Do highly cited researchers successfully use the web?" suggests that most of the most prolifically cited researchers do not have extensive social presences. 

At the same time, studies also show that using the social web can improve hit counts somewhat, which suggests overall that using social media can influence research, but may not have an enormous impact.

Now you'll have to excuse me while I go create a Twitter account for my research group. . . .