My best friend and ex boyfriend have feelings for eachother what should I do?

Technically who ever either of these two are interested in or date is none of your business and there really isn't anything that you can do other then accept it. I know it must be hard especially with your girl friend liking your ex. You could talk to her (in person not in writing) and tell her something like "...look Mary? I know that you are interested in Jim? but I have to tell you that you are putting me in an awkward situation considering you are by friend and he is my ex. I confided in you when i was dating him and even after the break up and now I feel that you are betraying me by dating him..." wait and see what she says. It sounds to me that she has already made her choice, him. Look you are all kids and she/you don't think clearly at that age. Although I'm sure she cares for you her attraction to him is overwhelming considering how out of balance teenage hormones are and friendships change and or end and the friendship you had with her might just be coming to and end. That doesn't mean that you have to hate or be mean, it just means it's over. I would also point out I think that you are jealous and I would strongly suggest not looking at their facebook pages because all you are going to do is upset yourself.