Google tells developers that Play Store apps can only update via the Play Store, Facebook doesn't respond
The change comes as part of the "Dangerous Products" section of the Google Play Developer Content Policy, and reads:
The core of the "Dangerous Products section has always been that Google doesn't allow "applications that cause users to unknowingly download or install applications from sources outside of Google Play." The idea being that any app or update that Google can't check through the Play Store could end up doing some unacceptable things.
Facebook has said previously that it is only pushing the updates to users who already have the sideloading settings allowed. Also, there were never any updates pushed without the user's knowledge, and users were always given the same options to see the new permissions when sideloading. Still, it's unclear what the company will do now. The policy is pretty clear though, so it doesn't look like Facebook has any choice.