Basically all the small stuff. Every user's identity, preferences, history, and connections. If you post text, that's there too. If you post media, that's somewhere else. Connections are the hard part, because it's a gigantic graph with the potential to make every query - e.g. to construct a user's timeline - hit vast numbers of servers. There's all sorts of special sauce to do with partitioning, caching, etc. to absorb as much as possible of that load before it hits the MySQL servers, but it's still a gigantic service even by Facebook standards.