In the early days of Facebook, when it was exclusively a social network for college students, the user interface was in constant flux. And young early adopters appreciated that cascade of new features and improvements, says Inspirit co-founder Julie Zhou, who scaled the Facebook app from 8 million to over 2 billion users as the company’s VP of design and research. But later, as the network expanded, users grew more sensitive to changes in the user interface. Facebook, in turn, became much more careful about changing features. Often, Zhuo observes, this meant breaking down a significant UI change into a series of smaller changes, and rolling them out over several months.