You shouldn't be using traditional database engines if you need genuine massively scalable capabilities.
Traditional SQL databases do scale pretty well (you'll find plenty of examples of big sites using MySQL, SQL Server or Orcale) but at a certain point you will hit the scalability limit. Exactly when this happens will depend on the level of demand, the complexity of your application requirements, how much you are willing to spend on expensive servers and how well you are able to optimise the database usage. Many sites will never hit this limit, but if you do then it's pretty painful and you will be faced with a big engineering challenge to fix the problem.
Beyond this point, you really need a fully distributed NoSQL database - e.g. something like Cassandra, which was in fact developed at Facebook to solve some of their big data challenges. NoSQL databases explicitly sacrifice some of the traditional SQL database capabilities (usually around transactions, locking and eventual consistency) in order to allow effectively unlimited horizontal scalability.
Overall my advice would be to figure out if you really need massive scalability, and choose your technology path accordingly.