Facebook usage has little headroom in U.S.

The U.S. lost 6 million users to wind up at 149.4 million at the end of May, said Inside Facebook. Why? Facebook is nearing the saturation point in developed markets.

Facebook is reportedly seeing traffic fall in its developed markets, but the news isn't all that surprising given that the social network is near its saturation point.

In other words, you could go into a football stadium ask everyone without a Facebook account to raise their hands and you'd be lucky to get a dozen people without an account.

Inside Facebook said its data shows that Facebook is nearing the 700 million user mark---687 million monthly actives at the beginning of June.

But growth for Facebook is slowing. The U.S. lost 6 million users to wind up at 149.4 million at the end of May, said Inside Facebook. In fact, May was the first time the U.S. lost users in the past year. Canada also lost Facebook users and the U.K. also saw a dip.

This data lines up with anecdotal evidence. Among the tech types that joined Facebook early, interest is clearly waning. Many folks on your friend list hop on Facebook, go update happy and then disappear.

In the U.S., eMarketer expects that online population---folks that go on the Internet at least once a month---to be 206 million. Facebook has roughly 72 percent of the U.S. online population signed up. Pew Research notes that 77 percent of adults in America use the Internet.

The bottom line is that Facebook is running out of headroom in developed markets to grow. The company will be increasingly dependent on emerging markets---isn't every for-profit venture?

In the long run, Facebook's challenges will be clear. The company will have to figure out how to increase engagement of its existing audience and ultimately extract more revenue from users.