That's a huge question - but in my view it really boils down to four things:
1) A clear sense of the problem they were trying to solve;
2) Crystal clear focus on only the most high-impact things;
3) Relentless pursuit of excellence in execution.
4) Luck.

That fourth one mightn't be a popular one to state, but the truth is that there are a lot of companies who try to do 1 through 3, but 4 always comes into play and Google is no different.

By monopolizing content and taxing access.

The promise of growth. And how crazy those dreams can be cast into people. Fundraising and valuations are a way of selling the future to people, today - but asking them to invest in tomorrow's value.

Interestingly I took a look at Google just a few weeks ago...

Google has three important things going for it:

1 - a single mission - >91% of revenue is from advertising
2 - dominance (monopoly) - Google has 70% market share for web-search (Microsoft 20%, Yahoo 10%)
3 - Advertising is a huge market that is migrating on-line

Also, by partnering with hardware manufacturers [Samsung, LG etc] it is trying to keep in front of the users and be the entry point of choice. iOS has 43% market share but Android has 49%.

The trajectory is onward and upward!

Happy to throw out some other opinions but that about captures it for me...