From your question, I can tell that you two were together in the past.
Your motivations for checking up on this are worth looking at, though, because it gives me the feeling that either something inside you feels like you don’t quite trust this guy or that you don’t trust the relationship you’re in to have trust as a quality (and so you’re always checking and testing because you don’t have that trust to begin with…
As such, some of the comments (which I have preserved) bring up points that I have since addressed in this revision.
Right off the top, you mentioned that you and he have agreed to be exclusive.
I'm in a two year relationship now and this is the first time anything like this happening to me.
But my point is, ignoring the issue will only make it worse. After a period, due to this ignorance, he may get the courage to bring this cheating out of the cyber world into real world. I think it is a GREAT idea to post your photo with the exact same profile as he has done on POF and see how he responds. Eventually one day I took out my photo album where there were many pictures of old boyfriends and casually flipped through it. If he comes off all defensive and accuses you of invading his privacy, you will know what kind if man you have on your hands - one who is not fully appreciative of you. You are the first prize, not the consolation prize. It's bringing me down and making me feel like there is something wrong with me, makes me feel super insecure.