There are 800 million active users on facebook. 800 million is a good percentage of the worlds population.
They are selling because they want the money.
Right. Just about everybody in the world who MIGHT want to be a FaceBook user, HAS ALREADY BECOME a FaceBook user. They are close to saturating their user market. Their revenue stream now is probably as high as it will go. So what justifies the enormous multiple over earnings?