it should be email. but you need to build a fabulous UX on top of it that is as simple and instant gratification as Facebook, and you need to do it essentially for free, or with a grant or something, because it will never make a dime.
I feel like reasonably effective retro-active deletion is an important feature of a social network, as it gives users more control.
No, there are middle grounds, such as a decentralized/local authorities. E.g. you send a hyperlink to the message on your server, rather than the message itself. Theoretically, you've still ceded control to the receiver, but practically, in most cases, you haven't.