as wmeredith replied below and I'll elaborate here:
Facebook has only one product, information. When companies want to grow, they have to produce more product. Facebook can produce more product in only three ways: getting more users, getting users to add more info, or sharing more of the info they've already added. There is no other way.
When new user accounts plateau, when users are content with the amount of info they've added or the additional value of that data declines due to signal-to-noise, the only method they have left is to share more of the information they already have in the form of greater public visibility.
Remember, Facebook's customers are not the users, they are the advertisers. If you want to establish a relationship with a company designed to make profit, profit should be the foundation of the relationship. Facebook's users do not profit. They provide labor to improve the value of facebook with no compensation in return, so the relationship is unbalanced. Even factory workers are paid, albeit not much comparatively, but someone else somewhere is making less and will migrate to the factory to fill jobs people quit.
In facebook's case, turnover can't be filled. The people won't come back and there is a diminishing quantity of new users to pick from. Eventually, users will stop joining because the knowledge we are sharing here will reach them before they do and they'll have lost their naive innocence -- a requirement of any sharecropper.
Why more sharing of information will make more money is because money is made through the information. Basically, advertisers can better target ads. By treating users badly, I mean in an increasingly selfish manner by doing things their users don't appreciate. Users don't want someone unilaterally deciding how much of their, what they consider, private information is shown to third parties -- millions of other parties -- like the FBI, Police, IRS, potential employers.