TITLE: Websites that promote co-operation and social networking among mathematicians
QUESTION [14 upvotes]: Are there some websites that could be defined as social networks for mathematicians and scientists? What I have in mind is something similar to Academia.edu or ResearchGate, but more specific towards mathematics and less formal ("Facebook-type", so to say): a place for discussing and sharing ideas without the "constraints" (so to say) of Q&A or forum formats. 
Also, in case that such places do exist, is there any evidence that this forms of cooperation have produced some results in terms of co-authored published papers, just like many cooperations born on M.SE and MathOverflow have?

REPLY [5 votes]: One of such websites is http://polymathprojects.org/. See, for example, a January 2015 blogpost by Peter Cameron about this project.
I suggest also to look at two preprints by Ursula Martin and Alison Pease: 

What does mathoverflow tell us about the production of mathematics?
Mathematical practice, crowdsourcing, and social machines

where one could find some further particulars relevant to the question, and also at the project "The Social Machine of Mathematics".
Update: came across an article from the Newsletter of the European Mathematical Society called The “Bounded Gaps between Primes” Polymath Project (the author is stated as D. H. J. Polymath).