The PE works out to be so high because there is a growth expectation from Facebook's one billion users. All that it says is that the market believes the chances that Facebook will be able to grow revenue, and in the future, profit, is very high.
PE is not an accurate indicator of the value of this stock. PE is used to compare mature yield stock, the type that all those Warren Buffet books talk about. With Facebook still being a young company the numbers to track are revenue growth and price/revenue.