Well, I'm back. We've just spent a very long day basically having it out over this whole issue. I've learned more as the day has gone on - some good, some that doesn't look so good.
I have asked for complete transparency (as much as that is possible these days) until I have regained a certain amount of trust. DH has agreed to this. He gave me his log-in names and passwords for email and FB and I have spent a good portion of the day looking through those accounts. Interestingly enough, the woman who I felt had become a little too friendly has been poking husband on FB (ok, I know that sounds naughty but those of you who do FB know what I mean.) Normally I wouldn't think anything of this. Lots of people he is friends with on there do that. But looking back thru his email notifications of activity on FB, I found that she had been doing this 15 to 20 times a day, everyday, all this past week.

Somebody needs to get a life. He said that he wasn't aware that she was doing it that often. The people I'm friends with don't do this a lot so I have no idea if it's something that would show up on his FB page repeatedly or not. And the email account this showed up in was not one that he uses very often so he had not seen all those notifications from FB. So I suppose it's possible that he wasn't aware she had been contacting him that often.
I also reactivated his FB account and have been going thru it. This allowed me not only to see more of what he has been up to, I was also able to go to the FB page of his little female friend and investigate her a little bit too. I was not impressed. My DH isn't the only one she has questionable conversations with on FB. He also volunteered the information that he asked her to friend me and wasn't aware that she had never done so. That and the whole poking thing may speak volumes about her, I think.
So I think that's it for now. We've agreed to some other things that we both think will help with the trust issue. Nothing terribly interesting on that so I won't bore you with the details. But I did want to come back and post an update and thank everyone again for their input and advice. You guys are great.