Spam detection.To detect someone suspicious is sending multiple friend requests or sending spam links
The Contacts which you see in chat are based on frequent contacts.
The order of the stories.The newsfeed is filled not just by the stories in the order of time or popular ones but it relates the people whom you talk frequently
Interest to specific ads may be seasoned. In advertising, one of the strategies is to maximize the click-through rate (CTR). The model for personalized ads should be able to learn online to adapt to changes efficiently.
Estimating the interest on New features.
Grouping multiple posts as one.For example When Steve Jobs died hundreds of my friends shared different pictures,quotes and videos as a tribute to his death.I got all those as a single post