Facebook's rate of growth is unsustainable and will top out around Summer, 2008. I think Facebook's traffic growth may be an illusion. I know that a great deal of my own traffic is web crawlers indexing my site and anticipation of Facebook's growth may be attracting robotic indexing through its API, creating a self-fulfilling prophecy but not one which has the value of human eyeballs. However, there may be quite a bit of marketing & advertising info which can be sliced and diced out of it.
As I predicted in April, 2006, MySpace.com has peaked out in Alexa traffic and Google inquiries. LiveJournal, a forerunner of MySpace, shows a net decrease in market share but has managed to retain a core of users. I believe that MySpace will repeat that pattern. Assuming that MySpace's peak represents maximum possible users and that MySpace will retain a core of users, you can make a rough determination of Facebook's peak.
My prediction - Facebook's Alexa traffic will hit an inflection point in the next 3-4 months and Google inquiries will peak sometime in Summer, 2008, effectively setting the upper boundary of Facebook's market.
