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.
Also of potential interest are these resources:
Academia.edu (another of these platforms) Although it seemed interesting, academia.edu doesn't provide the discussion forum style that researchgate offers.
arxiv.org (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)