If we operate under the assumption that with free services like facebook, twitter, instagram, gmail, etc -- can and will delete our content and revoke access to the service without warning at any point in time -- doesn't that make the service somewhat useless?

Most users operate under the assumption that there is some sort of guarantee that their account will continue to exist and they will have a consistent day-to-day experience. That is a BIG part that makes the service actually "useful".