To be clear, I do not think that Facebook itself is really a learning environment. It's a large, centralized piece of software that is getting creaky with use (we've seen more outages and the PHP code is once again dumping itself into users' browsers). Its privacy policies are questionable and it is giving out user information to applications willy-nilly. But it is still important, because it reveals many of the features future learning environments (and personal environments in general) will need to have. Something like the social network operating system, maybe. These are nicely captured by this article as Tony Karrer pulls together a number of recent resources on the site to throw out some ideas.