Don't worry, you haven't upset me :)
Lets play in fantasy land and say it does get that kind of traffic. I want to know if something like MySQL/Postgres/SQlite is up to the task? Sometimes you gotta spend money to make money, and I wouldn't want to head down a cheaper path if it would compromise the site and delivering a quality service to the people in any way.
"I want to know if something like MySQL/Postgres/SQlite is up to the task?"
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.
Yes, Digg uses MySQL and other optimizations.
http://www.computerworld.com/action/article.do?command=viewA...
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.