On Tuesday, Facebook changed its developer policies, ending the prohibition on apps that competed with the company’s own features.3. Facebook used a privacy app to collect usage data about its competitors.In 2013, Facebook acquired Onavo, an Israeli analytics company, announcing that Onavo’s tools “will help us provide better, more efficient mobile products.”One of those tools, an app called Onavo Protect, was especially helpful in helping Facebook sniff out potential competitors. The app, which was billed to users as a way to keep their internet browsing private, also collected data about which apps those people used the most — including apps not owned by Facebook — and fed that information back to Facebook.According to the emails released on Wednesday, Facebook executives received reports about the performance of rival apps, using data obtained through Onavo.Sometimes, those reports revealed up-and-coming competitors. One report included in the email cache, dated April 2013, said that WhatsApp, the mobile messaging app, was gaining steam. According to Onavo’s proprietary data, WhatsApp was being used to send 8.2 billion messages a day, whereas Facebook’s own mobile app was sending just 3.5 billion messages daily.Ten months later, Facebook announced that it was acquiring WhatsApp in a deal valued at $14 billion.In August, Facebook pulled Onavo Protect from the App Store, after Apple reportedly said that it violated the company’s privacy rules.4. Facebook executives wanted more social sharing, as long as it happened on Facebook.In November 2012, Mr. Zuckerberg sent a lengthy note to several top executives called “Platform Model Thoughts.” It outlined how intensely he wanted Facebook to be the center of everyone’s social life online.