An idea or information circulates when it moves from person to person and spreads widely.
When we say that news circulates quickly online, we mean that people are sharing and reading it fast. Think about social media - an interesting article or a funny video can spread from one friend's account to another in just minutes. If you have a rumor going around at school, it might circulate among your classmates until someone confirms the truth or puts the story to rest. Sometimes news or information gets stuck somewhere and doesn't really circulate much at all - like if there's a big controversy that lots of people don't want to talk about.
(intransitive) to move in circles or through a circuit
