Is he treating you differently since he's been back with her? Is your relationship with him healthy? I suggest you stick to what's relevant, especially since you admit that you just looked at his messages on a whim, and not because you had other evidence that he was hiding something from you or being deceptive (and even if that be the case, there are other avenues to explore without resorting to "self help" at the first opportunity). Do you think it's necessary that he know you snooped in his FB? If you truly do have a problem with what you read, then I venture the suggestion that there is something else bothering you that has nothing to do with the girlfriend, although she does make a convenient scapegoat.
But I don't know you and you didn't share many details, so I'm just running the last of the caffeine off the top of my head.
"Caffeine" doesn't follow the "i" before "e" except-after-"c" rule... I tried to do it like that and spell-check yelled at me.