Buying growth is rarely a cheap concept, especially for small tech companies that lack the pocketbooks of their much larger peers. 

Yelp knows this well. Founded 19 years ago as a pioneer in the field of online reviews for local businesses, Yelp has established an Internet advertising business now generating more than $1.2 billion a year in revenue. That is a 10-fold increase from when the company went public in 2012—and about what Google generates in ad revenue every two days.