iOS and Android mobile apps with a new "mobile first" attitude. And instead of one team working on the mobile development of the entire Facebook universe, each separate product team is responsible for its own mobile site. For example, the Facebook Messenger crew is responsible for its own mobile apps as is the team behind Facebook Photos. In the old days, one team would have been responsible for the mobile application for both products.
As a result, more people are turning to Facebook's mobile apps rather than using the browser. According to Nielsen, most social networks will soon turn to apps which will help improve the user experience across all social networks.