Messages are not stored, they are deleted as soon as they are downloaded from the server. So if you assume that energy cost is low and servers can be used for a

Messages are not stored, they are deleted as soon as they are downloaded from the server. So if you assume that energy cost is low and servers can be used for a long time, it feels like this could pay for lots of sent messages. They host everything themselves, so only the physical space for servers needs to be rented.