With more than 687 million users, Facebook is the undisputed social networking champion of the world, but it appears that it is slowly running out of steam.

For the second month in a row the number of new Facebook users has declined, indicating that growth is slowing down. Last year Facebook averaged more than 20 million new users a month, but in April and May 2011 the numbers dropped to 13.9 million and 11.8 million respectively.

However, with such high adoption rates, this comes as no surprise. There is a finite number of potential users and most people who wanted to sign up have already done so. Uncontacted  tribes in Papua New Guinea and Amazonia can hardly be expected to join Facebook anytime soon, but the number of users in developed nations also appears to be sliding.

For example, nearly six million Americans opted out of the social network, along with 1.5 million Canadians. The trend might have something to do with privacy concerns, or the fact that people are simply getting fed up with the fad.

More here.