If there's one thing Mark Zuckerberg has consistently excelled at as Facebook's CEO, it's destroying his enemies. When Google launched Google Plus, a rival social network, Zuckerberg put Facebook in a state of "lockdown" and informed employees that "Carthage must be destroyed," according to one tell all book from a former employee. After Snapchat began gaining traction several years ago, Zuckerberg tried to buy the company and, failing that, pushed out a long list of features and new apps to clone the disappearing messaging app. Google Plus is no longer viewed as a threat to Facebook. Snapchat is strugging to grow its audience. Now, Zuckerberg is turning his focus to a new enemy: Facebook itself. On Thursday, Facebook announced a major change to News Feed, its central feature, to prioritize posts from friends and family over posts from publishers and brands. The goal, according to Zuckerberg and Facebook, is to boost the "well-being" of the billions of people who use the social network by encouraging users to have more conversations and connections with peers, rather than being bombarded by brands. In short, Facebook wants to show the world that it can still be a positive force in peoples' lives -- or at least show that it's really, truly trying to be. Suddenly, the most existential threat Zuckerberg faces isn't some rival service. It's his own.