Summary, their oldest markets, i.e. US/Canada/Europe have reached "peak Facebook", and numbers are going down in those older markets. E.g. in the Technorati article...
> Data released by analytics firm SocialBakers suggests that people are
> leaving Facebook in their millions.
> It reveals that the social network has shed 6 million US visitors in the
> last month, which represents a 4% fall. The UK fares no better having
> lost 1.4 million users last month, a drop of 4.5%.
> Worryingly for Facebook this is far from a blip. In the last six months the site
> has lost 9 million users in America and 2 million in the UK. There's a similar
> picture across the developed world, with usage falling in Canada, Spain,
> France, Germany and Japan.
Yes, the numbers of well-off North Americans and Europeans leaving will be more than offset by the influx of third-worlders. But that guy or gal in the call centre in Mumbai, or the peasant in Asia, is not worth as much to advertisers as the westerners that they replace.