This is a nice layman's summary of the whole Facebook phenomenon. It's a private members club of 500 million people. The inherent threat to Facebook's existence is, therefore, a mass exodus. Social networks are nothing if not fickle, so to hedge against this Facebook has to wall the garden: members can put stuff in, but we can't take stuff out (easily, anyway). If I upload a photo to Facebook from my phone, I can't share that photo using Google Buzz.
If, like me, you believe that information fundamentally wants to be open, then Facebook may seem like the best enabler of that; except that it's a privately-owned company and it co-owns whatever information you give it. Your rights are not exactly violated so much as 'shared': you can control you stuff on there, but so can they. But they can also change the rules on that, and if past evidence is anything to go by, the majority of us won't care. So, Facebook does not really practice open information.
Unlike Google. Imagine if Google had thought of Facebook first. How would they have done it? I suspect it would have been rather like Diaspora. A completely open platform for anyone to create their own Facebook, but federated using Google identity management, and of course, linking to Google's giant advertising machine. Would it have caught on as well? Very doubtful: most of the awesome tech that Google develops is too raw, too geeky for mainstream users, and even the more user-friendly stuff assumes more tech knowledge than the average punter has.
So Facebook has its place. But as its importance increases, as it burrows its way into the social fabric, it will hit sensitive seams like religion and politics head-on, not just via its users, and those walls around the garden will be challenged. The question is: will they open them, or fortify them?