I don't know for sure, but AirBNB had the weird domain name airbednbreakfast.com or something, and it still has pretty weird company name too. It's not really what you would choose for disrupting the hotel industry.
By decades of media coverage, "Microsoft" evokes images of a behemoth. But try to imagine what the name "Microsoft" must have sounded like -- it doesn't sound very ambitious. It sounds a little lame. If you planned to be the biggest company in the world, would you name your company "Microsoft"?
Microsoft was started when Gates was 20 or something. I think a lot of 20 year olds are just trying to make something interesting. Some are dreaming of taking over the world, but those are the types that probably don't actually do it.
I think you can see a similar thing with Linux. If you look at Linus's original mail announcing Linux, it doesn't sound like he has any goals to run on the majority of servers, mobile phones, many desktops, and tons of embedded devices.
He even insisted it would never be portable past i386!!!
I agree with pg that a lot of these people are living in the future and don't necessarily realize that they're going to be riding this hockey stick growth. Even if you're doing it, I'm sure it's hard to predict. There's hundreds of decisions and improvements you have to make along the way. There's no one idea or feature to implement to get that big.