bumping into old flames on facebook ain't nothing lol... it's only when you have to click the "how I know this person" box that you begin to start thinking diplomatically
Most people say don't go out with old flames reasoning that if something didn't work the first time then it won't the second. If she bugged you for example and got on your last nerve then just because you're single now and want piece you're blinding yourself to the annoying things that will come back and annoy you again as you go on. Therefore most people say it's a waste of time.
I say don't make up rules for yourself like that. Given sufficient time people change and depending on how long ago it was and what happened in the interim people can change quite a bit or grow. Feel it out politely and see how it goes.