On the one hand everyone in the world is getting a smartphone since the pricing for chips has come down so much this year. That will make the market for mobile computing/commerce/you-name-it huge.
On the other hand, Facebook hasn't really nailed their business model. Yes, it's pulling in a lot of cash, but not enough to justify its valuation. Assume Facebook IPOs in 2012 and because everyone except newborns is on the network, the price gets bid sky-high, but they fail to grow revenue. Investors get scared and pull out and take the rest of the tech market with it. Then Facebook grows credits to the be the biggest payment platform in the world and the whole market goes up again.
Like PG said, market valuations are a sine wave. They are educated guesses, no one knows anything for sure.
Here's more on Facebook's valuation: http://community.nasdaq.com/News/2011-03/how-to-justify-face...