I would give fB a bit of a wide berth. Just to give you some clear headspace. He and his family knows you are a good person so they will do anything they can to win you back/give excuses. Don't give in!!
Here's a true story:
When I was 25, I was madly in love with 'The One'. We lived with each other a few years, convinced we were going to get married etc. I once came home to his work laptop open (he wasn't expecting me, I came home early from work ill). This was way before the likes of fb and maritalaffairs.com etc.... He was messaging other women and arranging meetups by the motorway using the then basic 'chat' / 'messenger' probably Yahoo.
10 years on, he contacted me out of the blue. He's now 'happily' married with 2 dcs. He messaged me saying he'd be 'up for a one night' stand that he thought about me loads, that he's love for a bit of fun. I thought I'd string it out - also for a bit of fun. Goading him, pretending I was interested. All the while, showing it to my dp for a laugh. I got him to book us 5* hotel, dinner, I got him to arrange a night away....LOL
I sent him a printed photo imaged transcript of our text messages and I asked if his wife would be interested in seeing it because obviously, she'd love to get in on the fun too? LOL. He got bloody abusive. I didn't tell his wife in the end, as his business is no business of mine - but quite honestly, I felt very bad for his wife having to live with a twunt of a cunt and I felt very lucky to have left him back when I did all those years ago. I am absolutely 100% convinced he is having an affairs now!