Wife getting too close to male online friend?

My wife has been talking to a guy on facebook she used to go to high school with. They started talking 8 months ago and contact each other at least 2 to 3 times a week. They mostly talk about mundane thing but they also share some intimate details about their lives and their spouses lives with each other. At this point I have only seen a couple of instances of playful flirting that were nothing really to get jealous about. I don't like the fact that my 32 year old wife and mother of my 2 kids shares intimate details of her life and my life with this guy. He is married to an alcoholic wife who has cheated on him and I don't fully trust his intentions even though nothing serious seems to be going on between them right now. I have always believed once you took the marital vows things like this were off limits. Am I overreacting?