Am I in the wrong for being mad about this like my husband says?

Ok so I know that the way I found this information out is not right on my part but I was looking at my husbands e-mail this morning and discovered a facebook message from a woman saying I had a great time talking to you last night I am working tonight so hopefully we can talk then. My husband works nights and apparently had called this woman several times while he was at work. Well I ask him who she is and why he is calling and being kinda sneaky about it and he first tells me she is a friend of a friend and tells me where she works then after some questioning he tells me she was a girlfriend at one time and he had called her because she had asked him to on facebook and it wasnt a big deal. So my question is: Is it ok for a married man/woman to call there ex’s and chit chat or is that stepping over the bounds? I think one of the major things that made me upset about it is I had never heard of this girl before today, he didnt tell me he had talked to her so It all seemed like a secret.