What is Mastodon, and why is it surging amid all the chaos at Twitter?

With Twitter in disarray since the world's richest person took control of it last week, Mastodon, a decentralized, open alternative from privacy-obsessed Germany, has seen a flood of new users.

"The bird is free," tweeted Tesla mogul Elon Musk when he completed his $44-billion acquisition of Twitter. But many free-speech advocates reacted with dismay to the prospect of the world's "town square" being controlled by one person and started looking for other options.

For the most part, Mastodon looks like Twitter, with hashtags, political back-and-forth and tech banter jostling for space with cat pictures.

But while Twitter and Facebook are controlled by one authority — a company — Mastodon is installed on thousands of computer servers, largely run by volunteer administrators who join their systems together in a federation.

People swap posts and links with others on their own server — or Mastodon "instance" — and also, almost as easily, with users on other servers across the growing network.

The fruit of six years' work by Eugen Rochko, a young German programmer, Mastodon was born of his desire to create a public sphere that was beyond the control of a single entity. That work is starting to pay off.

"We've hit 1,028,362 monthly active users across the network today," Rochko tooted — Mastodon's version of tweeting — on Monday. "That's pretty cool."

That is still tiny compared with his established rivals. Twitter reported 238 million daily active users who had seen an advertisement as of the second quarter of 2022. Facebook said it had 1.98 billion daily active users as of the third quarter.

But the jump in Mastodon users in a matter of days has still been startling.

"I've gotten more new followers on Mastodon in the last week than I have in the previous five years," Ethan Zuckerman, a social media expert at the University of Massachusetts at Amherst, wrote last week.

Before Musk completed the Twitter acquisition on Oct. 27, Mastodon's growth averaged 60-80 new users an hour, according to the widely cited Mastodon Users account. It showed 3,568 new registrations in one hour on Monday morning.

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Rochko started Mastodon in 2017, when rumours were spreading that PayPal founder and Musk ally Peter Thiel wanted to buy Twitter.

"A right-wing billionaire was going to buy a de facto public utility that isn't public," Rochko told Reuters earlier this year. "It's really important to have this global communications platform where you can learn what's happening in the world and chat to your friends. Why is that controlled by one company?"