It's these kind of forward projections that cause booms and busts, ie. people assuming that housing prices will increase at a steady/regular rate over time. Just as we saw that this doesn't happen in a steady way, nor will facebook steadily decline over the next 3 months.
Also check out Fred Wilsons response (http://www.avc.com/a_vc/2012/06/some-perspective.html). He makes some good points about the returns still being pretty damn good.