I feel you. My H had a facebook account that was just for him and OW. He even told me he would NEVER have a 'stupid fb account' and the entire time he had one. So fast forward to earlier this year. His cell phone stops texting, and for a few days we were communicating via FB, just like they did. At first I cringed, but then I realized - he is MY husband. She is gone. This is just a method to communicate. It helped, somewhat.
Also, the email acct he had that he set up the FB account I took over right after DDay as it had all of the "XXXX sent you a message on FB" along with what her messages were. He created a new account with the same name with a 1 at the end of it. He recently told his sister that he 'forgot the password to his account, and now has to use a stupid 1 at the end of it'. I wanted to scream "NO, I TOOK THAT WHORE ACCOUNT AND I STILL HAVE IT" but I just let it go.
These communication tools are really huge triggers. The only thing we can do, if anything, is to acknowledge they are just tools, nothing more.