Closed Social Media or messaging apps are social media platforms that are intended to have membership of specific individuals for secure communication. Some of these platforms are used to communicate within business work groups or social interests.
How do our individual social media feeds differ? It depends on what you view, what you like, what you comment on, who you follow, etc.
An Echo Chamber is usually a social media(s) environment where a person only encounters information or opinions that reflect and reinforce their own.
A filter bubble is the intellectual isolation that can occur when websites make use of algorithms to selectively assume the information a user would want to see, and then give information to the user according to this assumption.