The honest-to-goodness justification you're going to hear is that Facebook is a nascent company, the potential earnings of which are only loosely a function of current earnings. That's to be contrasted with the likes of Google, Apple, and others as those tech staples have more predictable future earnings potential, which is at least relatively more bounded by current streams of income from reliable businesses.

NOTE: This is not to say I think a P/E of 100 was remotely justified.

You shouldn't really look at stock price (it's an arbitrary number). You should instead look at Market Value - should FB be worth 80bn when Google is worth 200?