Re: Why facebook used mysql ?

In addition to the license a product is currently available under,
you need to also consider who owns its copyright; who owns
its test suite (which may not be open source at all); who
employs all the people who understand the code and who owns
the trademarks that identify the product.

Red Hat owns none of these things with respect to Linux
(although they do for various other products such as
their Directory Server and JBoss).