Hashing is not reversible so obviously it is not hashed. You hash data you want to compare later to see if it is still the same. For example you may hash user passwords you store in your database. So you don’t know the actual password, but can confirm later that the same password is still being used. You know or can infer someone is storing your passwords in plaintext when they have a maximum length as that indicates they are not correctly hashing.

It is however possible and even easy in many databases to do row or document level encryption. Many privacy first applications do client side keys and encryption so the database does in fact have no plain text in it.

That’s a good point and I don’t know the answer to that (my guess is encryption is involved), but as other people have pointed out, Facebook has an alternate encrypted messaging service, WhatsApp, so Facebook is clearly capable of not being able to access its users’ messages.