Facebook is interesting. I find the vast majority of the apps to be completely uninteresting -- my desire to "poke" on Facebook is very, very low -- but there is some really neat stuff going on with information-sharing apps like Visual Bookshelf.
Although the people who designed the UI for Visual Bookshelf should have their fingers broken. Okay, that's a bit harsh. They should, however, have it explained to them that HTML resizing of images is a really bad idea, and that book covers, which have words on them, should not be resized down into unreadability.
I also stumbled upon (or rather was sent a link to) Plaxo, which I am liking a bit more than FB. I have been a LinkedIn user for a couple of years now, and Plaxo is closer to that, with a more profesionally designed UI. Plaxo also lets me separate my profile between personal, family and professional segments, which lets me give my relatives my home address and phone number without sharing it will all of my business contacts.