Evidence to support this claim? Facebook has around 600 million active users today, giving them several billion more prospects. Their core product, the graph of human relationships, might very well be the single most valuable business product ever.
Several billion more? There are an estimated 2 billion internet users, and of those not using Facebook, a significant proportion of those are in countries that ban their population from accessing it.
Facebook might have plenty of scope for revenue generation, but their actual number of pageviews and users can't grow much more without them actually being the internet for the average person