Facebook has a very good reason to focus on the developing markets, it's long past the saturation point everywhere else. While the site is still seeing healthy growth in places like Brazil or India, where it's already quite popular, the situation is reversed in countries such as the UK, where Facebook was popular early on.
In fact, the latest figures
from SocialBakers show Facebook losing close to one million users in the last month alone in the UK.
There are a few caveats to the number, there's probably no reason to panic just yet. For one, the stats are based on the numbers Facebook provides to advertisers, which are an approximation of real usage.
For example, while Facebook has long since announced that it has more than one billion active users, the number tracked by SocialBakers hasn't crossed that milestone yet.
The numbers also don't include mobile usage for the most part. People also tend to use the internet less during the holidays, or at least they did in the olden days of laptops and desktops.
Still, it's a clear sign that Facebook can't push beyond the current usage numbers in many of the places it's popular once about half of the people in that country use the site.