Monitoring only works if it is a consensual thing that he asks for as a way to keep him accountable. A couple I know went through this thing a few years back, not exactly the same as your situation, but an internet thing that the man was addicted to. They fought a lot over it. He was always hiding his internet activity, and she found herself tracking him all the time, always suspicious, always disbelieving and wondering if he had been on again that day. They finally went to counseling, and with the help of the counselor, she was able to communicate– in a way that he understood– that his internet activity was a form of cheating on her. She felt she could not measure up, was not enough for him, that he didn't want her, and that she was sharing him when– as his wife, and he her husband– she shouldn't. He felt terrible, and wanted to change– though at first he didn't want any help doing it. As his own man, he wanted to prove that he could beat it himself. Unfortunately, as with all addictions, he couldn't, and after a few more tearful and hurtful incidents, he broke down and asked her to be his Accountability Partner. She purchased a Content Barrier program, which allowed her to create its settings according to their needs. As things changed, she was able to adjust those setting accordingly. At first he had no access to the internet unless she logged in, and she stayed while he checked his email and so forth. As time went on she was able to loosen the restrictions. They still use it to this day, and their marriage is one of the strongest and happiest I have ever seen.
The thing is, the Content Barrier only worked because he recognized he needed it. She didn't force it on him (though she wanted to). They went to counseling first, to deal with the problem, and figure out a solution.
Facebook has been great with connecting friends and family, but it definitely has its dangers, as any communication device does. The easier it is to connect, the easier it is to fall.
You NEED to go to counseling. You NEED to set up boundaries that he cannot cross. If that means restricting his internet activity to only when you are there in the room with him, so be it. But it has to be something you agree to together. Counseling is definitely a first, and a must. I would also say that Facebook, for now, be a Do Not.
There is a saying: Good Fences Make Good Neighbors.
This saying is SO true. When boundaries are clear and strong, people act and interact better because they know what is okay and what isn't. Good fences in a marriage is good, too. You are his wife. He should not be contacting old 'female' highschool friends, especially since he has a wandering eye.
Find a good counselor. It will be expensive, but there is no amount of money too great if it will save and improve your marriage.
I hope this helps!
God bless!
M. D