I don’t think the social networking side of things has ever been trialled enough on this for us to be able to judge. It would be really handy for me as Teacher Network Coordinator for Hertfordshire to be able to set up some sort of network of teachers in Hertfordshire on here for instance, but I have never found a means of doing that. Others like me have mused about using facebook and twitter, but they don’t seem appropriate. Anyone who uses those sort of things already has their own purposes for them.
The various ‘closed’ or simply unjoined groups that TalkPhysics has seem to be serving a similar function to Google+’s circles function at the moment, but the concept of Alice’s circles not having to overlap precisely with Bob’s circles seems a nice one. A PGCE student has different requirements from a Curriculum developer, for instance.
Goodness only knows what the coding would be like on that, though…