My gut reaction is that people like Scoble with "5000 friends" are abusing the service, and that when they combine that abuse with mass messaging, it's easy to see why Facebook would react.
If a friend posts a status update, and a friend of that friend comments, you see the comment. The likelihood you will see comments from some random person you will never meet increases by the sum of fake friends of all of your friends.
Even if you have say, 100 real friends, but each of them has ~5 fake friends, you are exposed to comments from 500 random strangers.
Replying to people who contacted you first is not mass messaging.