Connecting people to one another is not just Caterina Fake’s hobby — she has made it her career. As the cofounder of Flickr, the landmark photography site, Fake provided a place for shutterbugs to share their work; they have uploaded more than 4 billion pictures. It was a seminal service that helped launch the era of user-generated content, spurring entrepreneurs to build Web sites and businesses based on volunteer contributions. Now, with Hunch, Fake is back, and she’s using her social skills to tackle one of the most vexing problems in computer science: online recommendations.