I assume that rather than "built" what you really mean is "reaching critical mass" much quicker (?)
But I'll answer for both anyhow just in case.
In terms of getting built, the tools at our disposal (naturally) are much better than we've ever had before and there a amazing ecosystems and platforms out there for new ventures to tap into, which gives you immense execution capability right off the bat.
In terms of reaching critical mass far quicker, it's a few things
1. Again - Platforms, there are a number of platforms already available which facilitates capitalising on a large user base readily. Think Facebook login, or the Apple App Store. There's a huge user base already waiting. We never had that 15 years ago. New Start-Ups always had to build up their user base organically one by one. These days, get your strategy right and you have most of Facebook, Twitter, Google+, Mobile users ready at your disposal.
2. We only hear about the successful stories and not the not-so-successful ones. These companies that have reached a large critical mass so quickly are getting their execution right. They're getting their value delivery right early on and understanding what it takes to activate user virality and intelligently driving the growth.
The mixture of great strategy and platforms readily available for leverage - that combination - I think is what is creating these results.
At least - that's my observation.