Actually I personally know 4 friends whose hobbies either became a business for them or earned them extra income.
1. Founder of Nebula, also creator of VPN GOM (go away MDA). Started as a fun project in uni, now running as a full time business.
2. Founder of bike site ToGoParts.com, similarly started as an undergrad project to connect with fellow bikers. Now also a full time entrepreneur.
3. Female blogger who started blogging in uni days. She makes good money from advertisers.
4. Former fund manager, stopped work, got bored but likes to bake. Selling cakes now online.
I realized that you have a better chance of succeeding in business if the product evolved naturally from personal interest. People ask me all time what business to start, I always tell them to look within. The business I cofoundered and sold also stemmed from personal interest in the Internet.