What do you do when your husband keeps adding his ex girlfriend?

My husband is in the army and before he deployed I found out he was explicitly texting his ex. I was very upset about it and talked to both of them. I was feeling upset/threatened and asked him to delete her. Now, he's almost done with deployment and he added her back to his facebook. I know this is kind of stupid, but it really bothers me and I just don't know what to do about it. He's been gone most of our marriage and he's already been untrustworthy with this sort of thing before. what the hell do I do?