There are a lot of reasons: glitches with the Nasdaq system that slowed early orders, an IPO price that got a last-minute bump, and indications of lackluster demand from institutional investors. But the most important is the underlying concern and growing realization that Facebook just isn't worth $100 billion. [...]
That's not to say Facebook won't be worth $100 billion some day, but it doesn't deserve that valuation now. The company still doesn't quite know how to fully take advantage of its huge user base to generate revenue and is still tinkering around with different business models beyond its core display advertisement business.