What started as a social network for bookworms has turned into the largest site for readers and book recommendations with 65 million members, 2 billion books added, 68 million reviews. Goodreads leveraged crowdsourced reviews, a sticky offering (more you use it more valuable it is), and reaped the rewards when Amazon purchased them in 2013.
Goodreads
Goodreads: What are your friends reading?
Goodreads entered a market full of incumbents as varied as local libraries, Amazon reviews, and conversations between friends, and used network effects to create value for users, authors, Amazon, and more.