Well since I don’t have visibility into the inner workings of these companies, I can’t say what exactly they can/should do, but I also don’t accept the premise

Well since I don’t have visibility into the inner workings of these companies, I can’t say what exactly they can/should do, but I also don’t accept the premise that the only thing they could do is monitor all private communications. I don’t think it’s impossible that there might be some objective heuristic that could help flag potentially abusive accounts and then put some roadblocks in place to protect minors from those accounts.