Algorithm as a service. Their algorithm prioritizes sorting interests first which has allowed them to scale faster and more efficiently than traditional social media platforms which prioritizes follows and subscribers first.
The traditional style becomes an issue because you don't care about every topic that someone you follow on twitter cares about. For example, I like to follow certain people on twitter because I like to hear their opinions on technology, but not politics. This has created a content moderation nightmare for facebook, twitter, youtube, etc which are trying to invest tons of money into fixing this problem.