Remember when Gmail launched in 2004, and everyone said it was going to kill Hotmail, Yahoo!, and AOL? Six years later, and this chart shows pretty clearly that while gmail has grown, only AOL’s pageviews have fallen. The rest have held fairly steady.

So what’s everyone freaking out about?

Perhaps it’s that Facebook email will be Facebook first and email second. Facebook + email = Facebook.

In other words: their social model, however flawed, will apply.

In other other words: no more spam, unless the recipient has opted in first.

WAIT! STOP IT RIGHT THERE! If the idea of an opt-in-only world scares you, you’re thinking like a marketer and not like a user—or actually you’re thinking like a bad marketer, because the good marketers always think about what their recipients are thinking.

For users—actual human beings who receive and send and use email—this could be the coolest messaging feature since threading (except that most users still don’t have actual threading.)

And there’ll still be a place for marketers to do what they do; Facebook has shown again and again that they’ll happily sell their users’ data to whoever wants it. Users “like” your page or whatever it takes to opt in to receive your messages, and Facebook will sell you stats on whether they read it or not, how long they spent looking at it, whether or not they were wearing pants at the time—whatever you can imagine, and lots that you don’t want to.

This will be absolutely amazing for the many modern marketers who have no scruples about collecting ever-more-intrusive formerly-private data about people, and potentially life-destroying for the many marketers who have no scruples about spamming people. But why is anyone surprised? This is 2010, not 1995. The world has changed. Facebook qualms aside, the idea of using a user’s social graph to keep their inbox clean has been around for years.

My guess—not knowing anything more than anyone else at this point—is that we’ll finally get to try it out.