Only 3 countries in the world have more people than Facebook

The Facebook phenomenon continues. Yesterday Facebook announced that it has a whopping 300 million active users.
Three. Hundred. Million. Users.
For some perspective on how huge that is:

• There are only THREE countries in the entire world that have a population of more than 300 million. Those are China (1.33 billion), India (1.17 billion) and USA (307 million).
• Russia has a population of 142 million. That’s not even half of Facebook’s user base.
• Facebook has almost five times as many users as the entire population of the UK (62 million).

With Facebook’s rapid growth rate, it will soon have more users than there are people in the US. Which leads us to the following question . . .

How large can Facebook get?

There are almost 1.67 billion Internet users in the world according to InternetWorldStats. That would mean that approximately 18% of all Internet users in the world use Facebook, which is exceptional.
Even though there’s still plenty of room to grow, how much larger can Facebook realistically get?