Well, I'd sugggest to treat the plate like a face, you (hopefully) would not post a stranger's face online either, despite seeing countless of them every day. Sure, in itself it seems to be harmless but you never know what consequences this can have for an individual person... I believe everybody should have autonomy over their identity, which means all data that identifies them, such as the unique identifier of the vehicle that is likely exclusively used by them. I might be overly cautious here but I know that I'd be severely pissed if somebody shared my plate online :D