Did you allow their apps access to photos? That would likely be the largest sized data set on the phone. Maybe they also send similar data for ads. Constant scrolling of the "feed" might require a lot of that content to be on your phone so it could be caching things.
Meta/Facebook is known to slurp up all kinds of data off peoples phones - location data, listening to the mic, etc. has been alleged in the past, possibly proven in court. You could try only using their website which I believe puts them in a sandbox where they can't send/receive as much. Or uninstalling it and not using it would mean 0 traffic to them from your router.
