Facebook used HTML5 for a variety of political and technical reasons.
On iOS, using HTML5 decoupled deployment of functional changes from the App Store approval process. They could add a new feature on the servers and have it available on phones immediately. Certain parts of the new app still use HTML to allow new features without updating the whole app.
On Android, HTML5 let them support a large number of phones with fewer special cases to work around old versions of the OS and phone-specific bugs.
On iOS, using HTML5 decoupled deployment of functional changes from the App Store approval process. They could add a new feature on the servers and have it available on phones immediately. Certain parts of the new app still use HTML to allow new features without updating the whole app.
On Android, HTML5 let them support a large number of phones with fewer special cases to work around old versions of the OS and phone-specific bugs.