This is a syndication feed that takes all of the active listings in MORMLS and sends them to Realtor.com, Zillow and Trulia. The data feed goes out every 15 minutes so your listings are viewable by prospective clients / buyers shortly after you enter them into the MLS.
All listings go out in these feeds by default, but if you should not want a particular listing to be syndicated to these sites you can control that on the Broker Distribution tab when you input a listing.
These are some of the top searched sites for real estate both for buyers and sellers.
If your listing is not appearing on any of these three sites, you can contact them directly.
For Realtor.com, call 800-878-4166
For Zillow and Trulia, call 800-986-5410