My philosophy on stocks is when everyone and their mother is talking about how great a stock will be, chances are its massively overpriced. I feel that way on FB, however, I like their obsessive CEO and their future earnings potential. Although, it will be overpriced now, I tend to think they are just scratching the surface and you want to have a 5-10 year investment window with a tech company like this. I don't buy they are losing popularity. FB is fully entrenched in our lives, especially women's lives from what I can gather observing my wife, family and friends. They just need to figure out how to get more money from this popularity.
You will not be able to buy at the IPO price. My guess is that doubles, if not quadruples, before you will be able to buy. FB will be volatile stock and you will be best served to wait and buy on the first big pullback (price drop). If I was pretty intent on buying, what I would do is take a reasonable amount of money that my wife and I can look at and say "we don't mind losing all this money if it came down to it," then put 50% of that money in whenever I had the chance. Then after the first big pullback, do the next 25%, then after the 2nd big drop, drop your last 25%.
Tread very carefully and make sure you have an appetite for volatility and understand you may lose money.