Q. Open Access - Can sites like ResearchGate or Academia.edu be used for open access? Are they compliant sites for the HEFCE policy?

Social media sites like ResearchGate and Academia.edu can be used as viable Green Open Access routes, so long as the author's final version is used and not the published version.  Likewise, you have to be sure that any embargo period has ended before using such sites as they have no way of temporarily hiding anything.  It is this latter point that many publishers mostly object to.  Hydra will always hold the author's final version under restriction until the embargo is over.