I am already in PF as you can see, and already enjoying it :D
By the way, no: I am not talking about Online Communities such forums, blogs, sites.
I am talking about something like Facebook or Windows Live Spaces.
I heard about ResearchGate, but there are many many others.
The problem with ResearchGate is that I personally find it too 'job and pubblications oriented' and thus 'too much' for a simple enthusiast undergraduate.
Main question:
simply choose the most crowded (most known = more chances that people will have at least one account there) or the niche ones (way, way, way less people but everyone looking for a scientific exchange purpose)?