The value of a lot of these apparently lightweight B2C apps is derived primarily from the attention that they get from their users. If you can build something that gets attention, then that's worth a lot of money.

In a world where people are spending less and less time watching TV there is now a huge imbalance between corporations who have lots of money and want attention, and huge numbers of people with smartphones and limited attention spans. It's like a thunderstorm, all these electrons trying to get to Earth, then suddenly, Bang! Instagram. These valuations, and the developer salaries they fuel are just a by-product of all that money trying to get from point A to point B.

Google got huge by inventing one new channel for that money to flow through. And a pretty brute force one at that: You searched for "laptop" here are some ads for laptops... It's still ahead of everything else though, which is basically a re-implementation of the old TV campaigns but on the web. If someone could only figure out how to show you laptops just before you thought of searching for laptops, then they would be even bigger than Google.