They may say buy on bad news, but following that philosophy might have lead you to buy Enron and any number of companies where the bad news wasn't just temporary but a sign of real underlying problems.

You're also falling prey to anchoring bias. Because Facebook stock has previously been twice as much as it is now, it seems 'cheap', and a good deal.

What you really need to consider is the net value of Facebook as a company (by assessing its long term money making potential), and deciding whether its currently under or over valued.

I personally would never consider buying shares in FB because of the weird voting structure/rights. When I buy a piece of the company I want the corresponding votes to have my say in making sure that company is run in my best long term interests.