Adam Ostrow’s post on Mashable from Tuesday gives a nice rundown of the latest Facebook news – site traffic is down in the U.S.

Combine that with David Kaplan’s post on paidContent last week, about how Facebook is focusing on language translation and international growth, and we have a story.

The point? Facebook is trying to find a new group of users, because nobody in the U.S. likes it anymore.

And I generalize, of course. But with all their recent growth, and with 90% of the media still brown-nosing like crazy, shouldn’t the traffic still be healthy? And how about Fortune suggesting that American adults – who gave Facebook all this growth – are getting tired of it now, too?

Let’s back up. Having a college-only social network is a really, really good idea. And Facebook did it well, back when they did that. The 18-24 demographic is golden for advertisers, Facebook had relevant social networking content for students (being able to find other people in your classes? Good idea), and students actually liked it. But enough has never been enough for Zuckerberg and Facebook, and Z’s recent refusal to have a good product is leaving all but their newest users feeling betrayed.

First, some history, in convenient timeline form:
February 2004 – Facebook starts
September 2005 – They let in high school students, which I think was the beginning of the end. Their strongest selling point before then was being the anti-MySpace. Then they started becoming MySpace.
September 2006 – Open to everybody. Did they ever intend to keep it college-only? Yeah, I don’t think so.

So, every time Facebook starts to see a slowdown in growth, they find new people to push it to. They don’t try to strengthen their product, increase engagement, or improve the user experience – they look for a new group that can jump on Facebook as a fad to keep their growth numbers up.

No wonder nobody believes the valuation.