[kaybe]

A possibility could be if your email shows up in your friends' contact list which FB gets from their email accounts.

FB continuesly harrassing me to give them my email password - of all things to ask for! - is something I will never get. 'We'll check your contact list for friends! We won't save your password!' My email password! You wanna install a camera in my room while you're at it?

But some people apparently actually use the service, and thus FB can find out you exist and who you're friends with. They might also mention you etc.