Here's a list of academic/work-related platforms that I have active accounts on. Besides those, I sometimes use Twitter to post work-related things.

Google Scholar
Mendeley was bought by Elsevier. See this.

Also of potential interest are these resources: (another of these platforms) Although it seemed interesting, doesn't provide the discussion forum style that researchgate offers. (pre-print publishing)
DBLP (CS paper indexing)
CiteSeerX (automated paper indexing)
IACR (eprint archive for Cryptography)
ScholarlyOA (blog about distinguishing quality and dubious open access journals)