Be impressed by the number of PEOPLE on the Internet:
   [1]http://www.statista.com/.../number-of-internet-users.../

   3.18 billion.

   That's impressive. That's not likely to go anywhere but UP from
   here.

   But Facebook? It's just a big website. A very popular multiuser
   bulletin board service. But popularity is fickle.

   Also: consider this : If 1 billion are on facebook, what are the
   other 2 billion people that are NOT on facebook doing... if
   facebook really has one billion real people, which is unlikely.

References

   Visible links
   1. http://www.statista.com/statistics/273018/number-of-internet-users-worldwide/