Remember that period of one month before joining the B-School? When you are just lazying around in your home and some kind senior sends an invite to your batch's yahoo or google or FB group (depending upon number of grey hairs on your scalp). And the flood of emails that flow following a few awkward introductions in the beginning. And people asking all sorts of doubts ranging from need of mosquito net, to quality of mess food. If you don't believe me, just check out your group history, and you would be laughing your ass off now connecting the mails to real persons and friends you know.
Facebook lagrely is quite similar in nature, and we are currently in the period of initial hyperactivity. There is a new platform, and people are keen to explore it, projecting their best and active side. However, as we get used to the socially networked world, this initial euphoria would die down. People would stop informing the whole world when they visit a dentist, and when they go out for jog. And when that happens, the time spent on FB would come down gradually. So valuing a company on the basis of this period of super normal growth may not be the smartest thing to do. Thats where you need to apply everything real you learnt after you actually joined the B-School.