Right now you have no startup, you should go with the option that will allow you to launch the quickest and cheapest. Chances are - you won't see 1/10th the traffic digg or facebook do.
No trying to be mean - I'm being realistic. You can worry about scaling when the time comes - but it probably won't.
http://www.computerworld.com/action/article.do?command=viewA...
You haven't defined the task, other than to say it is a web application.
At least one of those databases is up to the task, if the task is, in fact, an application that requires a database. Some of the largest sites in the world use one, two, or all three of those databases in one way or another. The best designed of them probably use them appropriately, and thus may have a use for both SQLite and one of MySQL or PostgreSQL.