I received a few private emails yesterday on my facebook spam filtering post.
And what I took away from all of them is that social networking viral marketing is like email but not identical. Email is in some ways simpler. Either the message gets through or it doesn't.
Viral marketing in a social network is more complicated as there are multiple dials you can play with in a viral marketing program. Facebook implicitly recognizes this in that they have to regulate both notifications in the news feed and invites. Over time there will be more ways to get viral adoption in a social network.
And so the binary decision, let the mail through or not, is not what's going on. Yes, it's all about reputation, but no you can't simply whitelist or blacklist an app based on reputation.
Facebook needs to take a carrot and a stick approach to this problem. They should absolutely punish apps that have bad reputations. But they should also reward apps that have good reputations. Decrease the virality of apps of bad reputation and increase the virality of apps with good reputations.
As I said in my post yesterday, Facebook has been evolving the rules of the game for the past six months and they are getting better and better at this. I expect they'll continue to innovate and I hope that they'll implement some kind of reward system.