The new trend for social networks is to go niche, a way of new challengers carving themselves a place in the market. So from a marketing point of view its no surprise.
What's interesting is why we as users are so keen to close down the possibilities of such a great open space, surely defeating the point of the internet in the first place.
I can't decide whether its a good or bad thing to have closed networks. Maybe if we had more open or universal ones we could communicate on different sites without having to sign up and create new profiles all the time. It does seem to slow down the process and deter people from using more than a few different sites.
Ever time I fill in a registration form I get more conscious how much personal information I'm putting out there and whether it can be manipulated for someone else's' benefit. Maybe this is a good thing though as it makes us more careful, as privacy is always going to be an issue.
On the plus side of closed networks and possibly why the trend is moving towards niche network sites is because people want to feel they belong and having a closed group of people with similar interests creates a sense of community and a group identity that we want to associate ourselves with.
Speculation over facebook's drop in popularity I would have thought to be due to its decision to open up to the general public, eliminating its exclusivity to students which I think had damaged its cool factor and has increased the take up of applications due to the different type of people who participate on these networks.
The whole of society works on the basis of the in and out groups. Those who are in have power and superiority over those they don't let in but they have to let enough people in for it to be seen as a desirable and well known group. Therefore I think trends will always switch between open and closed areas to keep the balance right.