Websites that promote co-operation and social networking among mathematicians

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?

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).