What are you basing your decision to buy the Facebook shares? Have you bought shares before? How long do you intend to hold these shares?
These are the kinds of questions you should answer before deciding to buy any share. Buying a share/stock is really an investment. If you analyze the stock and think it meets your set criteria then buy the share. However, personally Facebook does not look like a good bet. There are many flaws in the the way they earn money and how long will it really last?
Forget that it's Facebook and appraise the company as you should for any type of investment. Have you had a look at their accounts? Can you see how they make their profits and how sustainable those figures are? Think like Warren Buffett and invest in companies that have sound financials that makes sense and you will find it hard to go wrong.