This article is large, it takes more than 10 minutes to be read in detail but it gives a very good perspective about the software companies over 1B$, the perspective of VC, consumer and enterprise oriented companies, how they are formed and some other data that provides a good perspective of the market.
http://techcrunch.com/2013/11/02/welcome-to-the-unicorn-club/
Who will be the next Unicorn?
WordPress
Google + 
Facebook