People always want to know how the winners won: how did Yelp create their marketplace? How did they solve the chicken and egg problem, populating their two-sided marketplace with buyers and sellers? 
They did it with good old fashioned business strategy, a la Good to Great by Jim Collins (you're a cave-person if you haven't read this), available at:
http://www.amazon.ca/gp/product/0066620996/ref=as_li_tf_tl?ie=UTF8&camp=15121&creative=330641&creativeASIN=0066620996&linkCode=as2&tag=networkextern-20