Facebook & MySQL

Its hard to believe that Facebook maybe is 15 billion worth, that's a lot of money for a relative new Web 2.0 company especially now that the US is in a slow down economic situation, but real numbers are hard to get when there's a bubble, I'm not an expert in web's bubble, but from the transaction between Sun and MySQL database engine for 1 billion, I can figure out that there is a bubble over there. Why is there to much difference??, MySQL is used by a lot of companies (thousands) while Facebook is a social network, the only answers that I have come up so far are that there is a Web 2.0 bubble now and that MySQL is not known by the end-user and Facebook is famous becuase is a front-user application, that could be two answers, for me is clear that MySQL is worth more than 1 billion, maybe I am wrong??