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.
Just to pick a nit. To date Facebook's P/E is high because Facebook said so. Facebook's value has never been set by an open market until about 3 or 4 hours ago.
Its P/E isn't mean to be an indicator of value, its shows the markets expectations of growth. And I think a lot of people are interested to see if the "market"'s expectations are inline with their own - and to that end P/E is great.