Unlike Erik, I don't think an open alternative to Facebook will emerge, the inertia at this point is too big and you would need a huge critical mass of people (and organizations) to make it useful. Hard to attain. The only contender on the long run to FB could be reddit, because they seem to be moving in that direction with the new profiles and so on. They have almost all the features that make a (general purpose) social network attractive, the amount of users, and the content.
Regarding the question if the WMF should build a social network for the masses, I don't think it should. A general purpose social network is mainly used for sharing personal events, viral stories, cat pictures, and so on. It does not offer long-term cultural value. A more interesting approach could be a niche social network, like a *social **learning network*. It is related to open knowledge, it offers some cultural value and it doesn't attract the same kind of idiocy that general networks attract. A social learning network could be oriented to life-long self-learning where users would share stories about what are they discovering each day, groups, creation of materials, etc. It could be said that users are already discovering new knowledge in our sites, but they have to go to other websites to talk about it... (for instance /r/wikipedia)
Another possible kind of network, could be one geared towards *governance and public oversight*. This is perhaps more interesting for governments, institutions and organizations, but still in the realm of the Wikimedia movement, because we also need some kind of social governance to build understanding and consensus both ways bottom-up, and top-down, and inter-organization. Not that we don't do it already, but perhaps with specific tools it would be easier.
Regards, Micru