so using social media

I have been with my SO for 2 and a half years and we've had problems with social media before (him talking to one particular girl that he'd previously been interested in) which led to us breaking up for about a month. He BEGGED me to give him another chance saying that he could be better. I figured it wasn't that serious, WRONG, but not that serious. So we got back together, this time with no social media accounts and everything has been great. We finally moved into our own home, got new vehicles, got better paying jobs, and he's really stepped up and I couldn't be more grateful for all the changes he made. Until, last night my sister sent me a screenshot while I was at work. He created a kik messenger account. I guess he wasn't thinking that all of his contacts that had the app would be notified. I just don't know what to say to him. I feel SO stupid for thinking that he had actually changed. We have been back together for seven months and no talk about social media so I am so confused as to why he felt the need to do this. What should I do? I found out yesterday and I haven't said anything yet..