The anti-spam movement has long known that blacklists aren't a very effective way to fight spam. The spammer just registers a new email account, or spoofs the From address, and your blacklist is worthless. Thanks to networks of 'zombie' computers, blocking IP addresses is ineffective as well. What's the only solution that eliminates all spam? Whitelists, of course.
Whitelists have a large number of drawbacks, though. Adhering to a strict whitelist means that users can't receive email from people who aren't on their list. That means you'll miss the emails from that old classmate or new business associate unless you set up some kind of cumbersome authentication system. That's generally more hassle that it's worth.
I realized the other day that whitelists are making a comeback, though, in an unexpected way. It turns out that more and more youngsters aren't using email at all. They prefer to message each other through Facebook.
Sure, the interfaces are clunky, it's hard to find old messages, etc, but the spam count is almost zero, since you can only message your friends. In essence, the social network acts as a whitelist for communications.
Sadly, this approach doesn't work well for adults. We communicate with people outside our immediate circle of friends quite a bit. We need to be able to shoot spreadsheets to our business contacts, or find that old email from our boss. For kids under the age of 18, though, none of that matters. They just talk to their limited group of friends, and for that kind of communication, social networks work pretty damn well.