this happened to me. I saw my husband's fantasy football friend as an email when I attempted to log into facebook. He has never been to our house. We don't even really know him. BUT, he did communicate with him on both of our gmail accounts, as I play too.
I have no idea why it showed up as a log in. My spidey sense went through the roof. But, why would he use another person's email for facebook? Must be some sort of bug.