I absolutely love this video of Zuck talking about Facebook back in 2005. If I had been in that room, I wouldn't have been able to guess that Facebook would become a $180+ billion company.
The best part: Zuck describes Facebook in minute 1 as an "online directory for colleges." Not only does that not like a billion dollar business, but it also sounds like a terrible startup idea.
The lesson: The key nugget is "I launched it in Harvard in early Feb 2004, and within a couple of weeks, 2/3 of the school had signed up". That is an incredibly strong signal that there's a really good initial product/market fit. This is a recurring theme that also permeated Y Combinator's recent Startup School, which talks about the importance of finding product/market fit above all else.
The kicker: I love how, in minute 3:55 Zuck is asked "And where are you taking Facebook?" He responds with "I mean, there doesn't necessarily have to be more."
Indeed... there didn't have to be, but there was. You don't need world domination planning if you can create world domination product/market fit. The rest takes care of itself.