I've been wondering what a fair value is -- *assuming* that FB is worth investing in at all, which is dubious at this point. But, for the sake of argument, let's say it is.
At IPO, the $38 was 107 times their annual earning, so that's roughly $0.35/share value, not counting their assets, IP, etc. Last I calculated, Apple had a P/E of 17, which is surprisingly low for a "hot tech property", but Apple has the earnings to fully support their valuation. During the past 3 years, FB has increased their revenues by 71% annually, or increasing it fivefold in this time (5x = 1.71^3). While I don't think that growth rate will continue, I do think it warrants a slightly higher P/E valuation of 20-25. Let's be generous and give 25. That in turn gives a valuation of $0.35*25 = $8.75/sh. No doubt these are hand-wavey arbitrary numbers here...but not unreasonable.
Last, my final adjustment is simply a value proposition. If I believe $8.75/sh is worth it (AND if their business model matures), then I need a buffer for a profit, at least 15%. So, my buy-in price is (1-0.15)*8.75 = $7.43.