I just thought of another potential challenge - SMTP limits. I think Gmail limits you to sending 40 messages per day for instance. This sucks if you have >100 friends.
Those limits are imposed by existing email software. The real underlying challenge you are getting at is dealing with spam. And that would be just as much a challenge with any distributed social network too, for more or less the same reasons.
Email did this badly from the start, because it started in a much more trusting world of people who did not spam one another. Authentication features were added later, but there was no requirement to use them, which limits their effectiveness.
Perhaps for a project starting fresh now, this can be handled better?
