What is an Instance on Decentralized Social Media?
An instance is typically the website you create your account with, which is built with a decentralized social media technology, such as Mastodon, Lemmy, or Peertube. It's a bit confusing, but there is Mastodon, the technology, and [mastodon.social], the instance. The same applies for all other decentralized technologies.
In the fediverse, users create accounts on instances, and instances communicate with each other, allowing users from any instance to access posts posted on any instance. On Bluesky, the custodian of your account's data is called a PDS (Personal Data Server).
Most fediverse-based social media technologies have hundreds or thousands of different instances. Some instances are managed by organizations, others are personal instances with only one single account. It's important to note that if you create an account with an instance run by an unethical person, all your data, including your private messages, will be in that administrator's hands. The same can be said for any website. The warning is still needed, however, because the idea of social media websites being run by just anybody is rather novel, as traditional social media have always been run by large organizations with plenty of lawyers to tell them what sort of laws they had to comply with. Meanwhile, a teenager can run a Mastodon instance nowadays for 5 dollars per month.
If the administrator doesn't pay to keep your instance running, the instance may disappear and with it all your posts. Consequently, despite all the clamor around "decentralization," the average person benefits from just joining the largest instance under the assumption that it's too big to fail.