Of course if someone was trying to hide from their parents then the parents would just have to take possession of the phone in question to view everything on it. Had a parent tell me that little caveat.
I was wondering because I keep an eye on the usage on one of my phones and noticed that there are messages on the phone back and forth but it does not register in my Verizon usage online
iMessages occur between two Apple devices inside the Message app. If you want to review facebook messages then you would have to be inside of the facebook app or website to do so.