It's more like email or irc. You downloaded a client, but you need a server to make your account on. Servers can come and go. If twitter or discord brings down

It's more like email or irc. You downloaded a client, but you need a server to make your account on. Servers can come and go. If twitter or discord brings down their service, discord servers stop existing and the client stops working. If a mastodon instance stops existing, you can use the same client and connect to a different one. It's my understanding that currently your account and data will be lost in that case.