Everything about Facebook is big. A half-billion users, 700 billion minutes a month spent there. It has 900 million pages, groups and events, according to the Facebook stats page. A new social networking startup, Path, is aiming the opposite direction — at least with its core functionality. The new San Francisco-based photo-sharing service, which launched as an iPhone app on Sunday, limits users to no more than 50 friends — barely a third of the 130 friends the average Facebook user has.