Also going deeper into this, one of the common trends is huge liquidity events which allow there to be a huge emergence of venture capital funds. The growth of tech stocks in the late 90's gave people tons of money to blow, and since they had trust in the tech economy since had made them tons of money, they further invested into tech.

Notice how massive growth of companies such as Google, Apple, Microsoft, and Amazon over the past 5 years. Additionally notice how many of the big VC's of today were early investors in companies such as FB. This gave them the capital and trust they needed to invest in all these random startups.

I bet that an Uber IPO will give VCs the money they need in order to continue to invest in more and more companies. The success of silicon valley is that every 8-10 years, there is a new company that reaches unprecedented valuations, and in turn people reinvest that money into the next generation of companies who will reach those same levels.