The primary indicator here will have to be volume. But I'm guessing somewhere between 10 and 14 dollars. Right now downward pressure volume is insane. Particularly insane today. Buying pressure isn't rivaling it at all. Supply and Demand dictates much lower prices.
Buy now. Even at $38 it will be seen as a good buy. After the first earning report in July the stock will be at $50 at least. Facebook has over 900 million users and that number is still growing. Now that they are public they will really focus on sales and will blow away July forecasts.
No one can call the bottom... if it goes into the teens before earnings, that is a good place to buy. Other than that I would wait until after earnings, buy half. Then in 2 months buy 25% more... Leave rest cash or buy protection (sell puts one year out)