Trying to build the next Facebook is a great thing. Declaring "I'm gonna build the next Facebook! It's gonna have millions of users!" and then abandoning your project after a month is a bad thing. Saying "I'm going to try to build the next Facebook" is an acceptable thing.
Ironically, this statement is neither humble nor realistic. No one cares about how you'd like them to be. If all entrepreneurs were humble and realistic, we'd still be in the Stone Age. Yea, a lot of wild ideas fail, but the small number of them that succeed usually have a big positive impact. It follows that society is better off with a bunch of crazy, unrealistic entrepreneurs trying to "build the next Facebook".