500 million tweets are published every day, but if you go to Twitter you will only see a small subset of these. Every platform has its own filter system to decide which content you see.
On Twitter you see the latest tweets from people you follow (minus those who have been banned), some sponsored tweets, with a “While you were away...” section at the top generated by unknown means. People are sometimes banned entirely from the platform.
On Reddit, you see content from subreddits you are subscribed to. Typically ordered by an opaque algorithm called “hot”. Users can vote, but there is no real way to see how this affects what content is displayed. Large sections of the site will sometimes disappear.
On Medium content creators are sometimes ordered to change their content under threat of it being removed altogether.
Each of these communities is siloed from each other. They control the filter bubble within their platform.
Link is fully programmable at every level. Anyone can take existing content on the system and develop new ways to select which content each user should see, or even utilize data from another filter bubble, for example analysing existing votes in a new way.