As an outsider watching all the action FB is not a fair price or undervalued stock based on what they are currently delivering. So you need to know what is your risk appetite and how long you want to stay invested.
Short term I do not see FB share providing tremendous value (except the volatility that may be there).
Long term you need to bet on certain things they could do right and if you can stay invested for a long time trusting that a bunch of smart folks will figure out something of value.
how are they going to leverage their user base to venture into newer forms of search on various new devices?
What will their mobile strategy be? Will they acquire companies, say, the second biggest player in mobile ads space after google?
Would they venture into FB phone (may be of tremendous value considering it fits into their social portfolio)?
So it is a lot of ifs & buts similar to any other startup investment, except that they are not a startup anymore and they are going to have truck loads of money to execute.
FWIW, these are points I would consider before investing in FB. :)
Also, the Greek debt crisis is another factor to think about. It seems like they have come to an 'agreement', sort of, as of right now. But things are still very much uncertain. Analysts are expecting things to smooth out, which is reflected in the prices, but if they don't, the world economy will tank. When it does, money will flock to the defensive stocks (consumer staples, healthcare, utilities), and money will funnel out of the cyclical ones (tech, heavy equipment, etc)
Then again, if you can get in this round, I'd buy.
To invest, you need to have a fundamental belief about the long term prospects of the company above that of the market. I think better answers would be "I think Facebook is a $150b company, so yes" or "I am 50% more bullish on Facebook than the market" or "I think I will be able to sell to people in the future who will be 50% more bullish on Facebook at that time."