Comparing the cost multiple for a private company's acquisition to a P/E ratio of a public stock is way different and not really a fair comparison.
Generally the starting point for any tech acquisition is 3x - 5x revenue multiplier. Occulus is hardware so their margins are lower than software, pushing them closer to the 3x mark.
It's far from precise but I'd maintain it's useful to illustrate that if Oculus is a success, Facebook made a great buy.