The biggest reason Facebook has a 100 Billion valuation is because people expect ad revenue to grow rapidly. If there are signs that's slowing down now, before they even go public, it has a huge impact on their valuation. 100 Billion is a very generous valuation, even if you expect Facebook to have tremendous growth, it's an outrageous valuation if you expect them to stay the same (or shrink).
Well if they do manage to do 5B$ revenue this year 100B would be a 20x multiplier on their revenue. Seems in line with other tech companies. But we'll have to wait until they start pushing out those quarterly reports to be sure.
