Twitter has 217 million daily active users: https://www.omnicoreagency.com/twitter-statistics/

I can't think of any other open platform with that large of an audience.

As for its relevance, while it may not host a significant share of the world's population, it enables the userbase to converge on messages very quickly, to coordinate social and political action, which allows them to have an outsized influence on the world. Twitter users don't have to make up the majority of the world's population to be able to give movements the critical mass of supporters they need to succeed.