@Eve32,
I'm with TomTomBinks on this one. I don't get why you would want to be with a guy who spends all his time on social media. Are you currently living together? Or are you living apart since becoming ex-fiances?
As an aside, just because he won't friend you does not mean he is cheating. It simply means he does not want to fully let you into his life. But after 12 years of being together, this seems a bit strange to me.
Bottom line, whether he is cheating or not (or looking to cheat or not), you have to decide if you want to live with someone who does not seem totally invested in you and with whom you have not felt connected to for a couple of years.