There's a bunch of websites running software that let's them mirror content from each other at users' request. Some of those websites are made to look like a Microblog, like Twttier. Some are made to look like Reddit. Some are made to look like YouTube. Some are made to look like Instagram. But they all speak the same language, and they can all share content between them.
That's the whole ecosystem here in a nutshell. Join one website, see shit from thousands of others as well as the one you joined. Depending on the UI, you may not have any idea that any of that content came from different websites.