I use Academia.edu much as I use LinkedIn -- a means of tracking down library users whose contact details are fading from Cambridge University's corporate memory. Both sites fulfil that purpose with far less distraction than Facebook; Academia.edu with perhaps the less distraction of the two, and I have the impression, not confirmed by a rigorous counting exercise, that many of the academics I've found there have not posted on the site for some years.
I have joined Researchgate on account of Kirsten's mention of its facility to request copies. That might be useful, to Haddon Library users if not to me personally. I may, in time, come to use Researchgate also for tracking people down, though its smaller membership makes that less likely.