My wife left for another man?

I just found out this morning. We hadn't been arguing or anything our marriage appeared fine (obviously it wasn't) but Thursday she said she wanted to separate because she didn't want the responsibility of marriage right now and moved in with her brother that night. So I accepted that and I left her alone giving her the space she wanted I didn't text her or anything unless either I actually did need something or she text me. She had something of mine that I was needing so I asked if she was around for me to pick it up and said she wouldn't be back till today because she was staying with one of her girlfriend's this weekend. Her brother called me a few minutes ago and said she came by this morning but he was still asleep. That raised my suspicion because her friend lives about 45 minutes away close to my wife's job.So I got on facebook and found her profile (I have an account that I deactivated a long time ago) and seen a post she made nothing sexual just that her and Brian had a good time this weekend and that she was going to try to find out what time he got off work today so she could introduce him to her brother. Since I'm obviously a glutton for punishment I went back and looked again and she had deleted the post. My guess is she got worried that I might take a peak. She has left her status still as "married" as well. So right now I know my wife went outside our marriage and she doesn't know that I know. I don't really know how to bring this up to her without making things worse than they already are. I had previously asked her before she left if there was someone else and she said no and even swore on her mother's grave there wasn't! So I already know that unless I catch her with the guy she is going to lie to me about it. Her brother and I are close but he won't tell me that because he doesn't want me more hurt about all this than I already am and doesn't want his relationship with his sister messed up which is understandable. I feel I deserve to know the truth. Any suggestions on how to get the truth out of her? Sure I could simply say hey I looked at facebook and that will in turn make her start asking me about that and try to take the focus off of her and this bs and try to put it all on me.