Reddit and Lemmy are topic-oriented systems: most users find things to read by looking for topics they're interested in. Despite the name "communities", these are basically topic categories.

Facebook and Twitter are people-oriented systems: most users find things to read by telling the site who their friends are (or which celebrities they want to get parasocial with), and looking at things from their friends or recommended based on friend connections.

On a site with topic-oriented standards, it's often kinda creepy to follow people from one topic to another.

On a site with people-oriented standards, it's often kinda rude to show up in distant friends-of-friends-of-friends' mentions to tell them your enthusiastically held opinions on the topic they happen to be discussing.