I think you need to find out what’s behind it. Maybe the person uses it for work. Maybe he/she uses it to keep up with friends/relatives abroad, and considers it only for this purpose. Maybe his/her little sister set it up and he/she isn’t even aware he/she has an account. You never know. I remember whan I was dating my husband I was obsessed with his Kippa. It was too close to the top of his head. I couldn’t handle it, and I couldn’t for the life of me understand what it was doing there. We were both very Yeshivish, why was his kippa so close to his forehead?!
Now I look back and laugh. when I brought it up with him, he couldn’t understand what I was talking about. My husband is a creature of comfort, that’s where his kippa felt comfortable, so that was that. He also doesn’t get style norms so much, so he didn’t really understand what all the fuss about kippa placement was about. End of story – we were still on the same page hashgafically, no matter where his kippa was!
Moral – talk to him/her, and find out why he/she has a facebook account.