I have to admit, I do think FB is a relationship minefield.
I am FB friends with my ex-boyfriend from last year - but have to admit, it still pains me sometimes to see his girlfriend on there, and I am careful not to comment too much or right after her.
Last fall I had a long-distance admirer who friended me first on FB, then started wooing me. He would post inappropriately intimate things on my wall (not nasty, but just lovey-dovey stuff that wasn't appropriate to our situation - should have been in private messages) and it was just awkward.
I ended the flirtation with him once I figured out he was - umm - a little weird, but for professional reasons, felt I had to tread lightly and couldn't unfriend him. So I tiptoed around on my own FB page, afraid to say much,for months until he finally unfriended me (thank you god!)
The whole experience made me very wary of having any currently active boyfriend as my FB friend.