Around the internet there are several social media and online reference managers that are more of less of interest to academics: Google Scholar, Academia.edu, ResearchGate, Mendeley, LinkedIn, Facebook etc.
At the moment, among them, only Google Scholar and Facebook have specific tags; the others have been divided between the more general tags social-media and reference-managers.
Google Scholar is clearly academic-oriented and in many fields it is relatively widespread, but probably cannot be easily classified either as a social media or a reference manager.
Facebook is a popular social media, not specifically geared toward academics, and it's explicitly mentioned in the description of social-media. Moreover, facebook has, to date, only 6 questions.
ResearchGate, which is explicitly mentioned in the social-media description, has a quite controversial status among the academics, as outlined by several questions and answers in this community (I suspect that many academics use ResearchGate in a passive way: after having signed up, they let their profiles live their autonomous lives.):
- ResearchGate: an asset or a waste of time?
- Should I send a "cease-and-desist" letter to ResearchGate?
- Is it legal to add your publications to ResearchGate?
In addition, at the moment, there are 51 questions containing ResearchGate and 47 questions containing LinkedIn.
After this long preamble, my stance is the following: either we create a tag for each website or we don't, but having specific tags for websites already covered by more general tags looks inconsistent.
Therefore, my suggestion is: keep google-scholar (because of 3rd paragraph above), burninate facebook, and don't create ResearchGate.
Disclaimer: my suggestion above is biased by the fact that, as I confessed elsewhere, I'm not that passionate about tags. Probably, users who use tags extensively, like the OP, might have a completely different opinion.