They buy the people not the company. Google and facebook need developers. Not the college graduate developers that have only done school project in C/C++ and never manage anything that people want in their life. They need people like the ones who made path because they know the people who made path understand mobile, understand design, and understand how to gain hype. It wasn't luck these guys were on tech crunch they understood their market. That is what is worth 100 million. Some people are academics, but only real test for a great app developer is can you make something people want.