Some might wonder if Facebook will ever reach 1 billion users, however Mark Zuckerberg is confident about it. And I think he is right, Facebook is the platform that can grow to this number an even finance its growth.
Since Facebook is thriving on information that users provide (your data is their biggest asset), they might even want to make sure that with the number of new users joining, they still acquire more and more information of their users. Facebook will earn more money if they have more data (and they might already earn around 1 billion a year), so why shouldn't Facebook just pay every user who completes his profile.
By not only offering the incentive of keeping in touch with your friends and catching up with old friends and like minds, but by also offering some money ($5?) for a completed profile. It would be easy for Facebook to get to 1 billion members and collecting more information from its users. Both the amount of users as the immense data set will push the revenues of Facebook and will increase the dominance of Facebook on the Web.
However entering data in a profile is easy, though incorrect information will devalue Facebook. Data validation should be just another part of the social ecosystem of Facebook. For example: your friends can validate your profile, making sure your will get your $5 dollars from Facebook (maybe even in micropayments for you and your friends per 'slice' of data you provide and is approved). Or: your friends can provide the information to Facebook and you can validate this information and approve the publication of the data.
Using this process Facebook gets more interconnected, creating more activity on the platform and it might even create more relations. And again, even this process will increase the value of Facebook. So paying $5 per complete profile might be just a bargain for Facebook.