Facebook autonatically logged on with someone else's phone???

A

AC Question

My bf was using his Android phone to Google, and clicked on a business link that happened to be their business Facebook page. He does not have a FB app or use Facebook on the mobile site or anything. He said it opened up under my account, and he could basically access FB as me. I have never logged onto my Facebook on his phone, I'm certain. I'm totally confused, how could this happen? Thoughts?