When you sign up for and browse this forum, some data is kept in the server logs. Because that's how the thing works, and it's necessary for it to work and be able to fight spam and so on. The logs aren't kept for very long (they take up space I can use for other things), and I'll never sell or otherwise part with any of the information to anyone else. Why would I?
I also use Google Analytics because I'm curious about the number of visitors to the forum over time and things like that.