Via Figshare, I’ve published to of the key figures I find myself using frequently in relation to academic social networking sites. Figshare issues Digital Object Identifiers (DOIs) to resources hosted on the site, so if you do find them useful, this is a handy way to cite them.
The first figure is from my thesis; a timeline of academic social networking sites (DOI: https://doi.org/10.6084/m9.figshare.1460787.v1):
The second is a chart showing the number of registered users on Academia.edu and ResearchGate over time. The data is drawn from online articles about the sites, and on Figshare, the figure is accompanied by a spreadsheet with links to the sources for all the data points included (DOI: https://doi.org/10.6084/m9.figshare.4769815.v1):