Often stocks are characterized as either 'Buys' or 'Sells'. But an investor can always chose to do neither. An investor can, in the words of Warren Buffett, "wait for the fat pitch". Basically that means waiting for great companies to trade at cheap valuations. If one cannot decide if a company is great and is trading at a cheap valuation, sitting with cash is perfectly acceptable.
Facebook is going public at around a $100 billion valuation. One can argue if that is a high or fair valuation, but it is not cheap. For investors that means that they can decide to neither buy nor sell Facebook and look for another company to invest in.
As Facebook grows revenue and profit over-time and the stock price fluctuate, the valuation will change. If the valuation at some point looks cheap the investor can buy Facebook stock at that time. In the meantime there are thousands of other companies to invest in.