I just had FaceBook say I "might know" my realtor and one of my tenants. The realtor could have been because he's in my email contacts. My tenant I just added to my phone. How would they know that?
When you installed the Facebook app on your phone and automatically hit 'Allow', the prompt was asking if you were OK with the application accessing your contacts.
They probably have either their phone number tied into their Facebook account, or have the Facebook app installed on their phone (of which the application pulled their phone # into the database).