Mastodon's strength, and weakness, is that accounts and posts are stored on a single server. If that server goes away so does your account and everything you've posted. This is good if that's what you want, but bad if you don't want it to happen. Unlike Twitter or Reddit it's not meant to be a one stop place for all things social. It's intended to give people a way to communicate with a small group of people easily rather than everybody in the world being able to see it. Servers can choose to remain private so people on other servers can't see their posts, which is great for privacy.