It strikes me as a bad plan to depend on 400% growth that's sustainable over a decade. You're right, people are horrible at taking growth variables into account. They usually overestimate them.

Facebook may well achieve the numbers outlined above. But let's not kid ourselves. People were doing the same back of the envelope calculations for MySpace when it first started to get big. I see nothing wrong with a judicious use of common sense and caution in this case.