> As for spam problems, require payment to add new contacts above the limit, and disable messaging to non-contacts. Or restrict messaging based on country/city (so that messaging to a different country is paid).

This just a) increases the costs for attackers, which don't actually stop them; and b) means the poor amongst a population will be limited in who they can talk to. Very convenient, that. Don't want your peasants talking to citizens from other countries.