A friend had a similar experience, except that he didn't even try to sign up for Facebook. His email is first name + last name. @ gmail and apparently someone signed up with his email on accident. After that my friend started getting lots of automatic friend requests from people he knew. He was able get the password sent to him, since it was his email afterall, and canceled the account. Like the OP he was pretty freaked out about how much Facebook seemed to know based solely on his email.