Johns Hopkins & CTY has had something called Cogito for several years that is an online forum and chat application for gifted kids. It was a lifeline for my D when she was in middle school (she just graduated from high school). She is still in touch with some of the kids she met there, one IRL and a few on other social networking sites now, although she no longer uses Cogito.
Here is a link. My D was invited to join based on her SAT scores submitted to CTY when she was in 7th grade. I don't know if you can just ask to join or not, but contact info is on the website. By the way, don't be put off by the Templeton sponsorship -- my D said there was never any attempt to stifle or direct their discussions in any way on the forums.
http://cty.jhu.edu/cogito/
