Editor's note: This is the last of a four-part series on how online home listings have changed the real estate industry. (See Part 1: Listings on the Web: From radical idea to ubiquity, Part 2: Online listings attract horde of innovators and Part 3: The risks of online home listings.) Every month from around the globe 25 million people click on real estate URLs and search millions of homes on the Internet. The number of Web sites offering direct access to the MLS has proliferated nearly as fast as the number of people searching the Web for houses. Thousands of agents, brokers, technology companies, lead businesses, lenders and the venerable Realtor.com offer direct access to the entire MLS online. The MLS genie is out of the bottle and there is no turning back. The event has create...