I just downloaded the app. Videos don't play automatically. You have to click the 'play' button. Doesn't look like it repeats once you play either.
"Based on how they were being positioned for the summer launch, video ads will appear to targeted users in their news feeds up to three times on the day they’re slotted and will begin silently playing when a user scrolls over them, according to source who heard Facebook’s pitch.
Audio won’t be activated unless a user clicks on the 15-second ad, at which point it will restart and spread over the right- and left-hand rails of the page."