I don't use Facebook at all, but I think I have at least three accounts.
The first is an account they created when my friends signed up for Facebook and let it run through their email accounts looking for contacts. They checked a box next to my name, which Facebook took as a signal to create an account for me and periodically send me a message letting me know that I've been invited to their service.
The second account is for my work address. Someone with the same first initial as me who isn't very good at using the Internet created a Facebook account and entered my email address. Facebook sent me a couple messages letting me know I had one more step I needed to complete to start using the service. After a while, they switched modes. That account is in an experiment wherein they email me every day. The subject has the names of three people I might know on Facebook and the body has three more names. The number of names in the subject that I recognize is remarkably high for a big company, so I think the Facebook app on Android uploads your phone's Contacts list (i.e. the people you email occasionally) to Facebook.
I just learned about a third Facebook account I have. I received two emails and a LinkedIn friend request from a Facebook recruiter this morning within the span of one minute. The first email was sent to my personal email account (not the one printed on my résumé) and the address firstname.lastname@example.org. That's pretty clearly a 64-bit integer, which is what a database like Facebook's would use as a user ID. I sent a test message to that address and it wasn't delivered to me, so I don't think it's meant as an alias for delivering mail to me. Maybe mail sent there gets appended to a communication log on my Facebook Job Prospect account.
Some people wonder what Facebook does with all the data about themselves that they give it.
I wonder what Facebook does with all the data about me that I never gave it.
Anyone want to guess how many accounts Facebook has about you?