FUD is actually useful (engineering FUD rather than marketing FUD) ... if you pick MySQL because it's easy to get started, you need to know its pros and cons. Those scaling MySQL at the biggest Internet properties in the world ARE database experts - Facebook for instance has a custom cache and memcached in front of their MySQL instances [1], shards their data and also uses other database technologies where appropriate. People like Facebook’s Mark Callaghan will be successful using any reasonable database.

[1] - http://gigaom.com/2011/12/06/facebook-shares-some-secrets-on...