On iPhone the problem was that FB left a bug (according to FB) in the application which made it play silent audio in background. Incidentally this meant that the app was able to bypass the iOS restrictions on background processes.
Whether it really was a bug or intentional is a subject of debate though.
On iOS your application can not stay fully active for more than 10 minutes. After that time it gets into suspended state and can be waken up only when it receives a notification (and the wake up time also has time limitations). There are only three exceptions to this rule: active GPS usage for maps and trackers, active voice calling apps (for ringing) and music apps that are actually playing audio. Silent audio thus enables Facebook to stay alive forever.
This had also the effect that when you were listening to music or a podcast and opened the FB app it would stop playing your audio.
Whether it really was a bug or intentional is a subject of debate though.