Quora is annoying as hell. You need to log in to see the answers or do anything. Talked to one of the dev why they do this and he sorta dodge the question, I got the hint so I left it as is. Quora is useless to me in general and Stackoverflow is much better.

Path, I've heard of path and I don't care for it. I've moved away from Facebook and hardly use it anymore. So yeah, the mass migration of Facebook and other social site is probably a trend.

Foursquare, I admire it a lot. I love the business model and the app is pretty decent, better than Yelp that's for sure.