Mastodon feels like the old internet. “Welcome to Mastodon, where you can boost a toot from hellsite.site to mas.to, but remember to CW politics and boot doxers or your instance might be defederated” is a sentence that will make sense eventually – but is unlikely to mean anything on your first day. Social media startups are ten a penny, but few are so proudly distinct from the competition as the countercultural network that has gained millions of new users over the past week as Elon Musk triggers an exodus from Twitter. Mastodon isn’t one site. Instead, it’s a protocol, a system of rules for spinning up your own social network that can also interact with any other following the same code. Some of those social networks are large and general-interest: Mastodon.social, set up by Eugen Rochko, the German software developer who first created the Mastodon protocol, has 169,000 users. Others are the opposite: hellsite.site, set up by Mastodon user @goat, has 440 proudly “shitposting” members and the slogan “be gay do crimes”. On the surface, any given Mastodon site, or “instance” as they are known, looks and feels like slightly tweaked version of Twitter. Users make posts (affectionately called toots, not tweets) that are short (although typically 500 characters, not 280), and can be reshared (“boosted” not “retweeted”) and replied to. There are a few additional features, such as a content warning (CW) option that lets you hide posts behind a caveat – as useful for movie spoilers or niche rants as for objectionable content or upsetting material. But the way the network functions is radically different. Each instance can link to any other, and users are free to follow posters on their own instances or across the wider “fediverse”. Administrators make and enforce the rules on their own instances; on a larger one, that might be a full-time job, while on a smaller instance, it is no more work than being in charge of a mid-size WhatsApp group chat. And the rules can be as tight or as loose as they want. Rochko’s original Mastodon instance, for example, bans “racism, sexism, homophobia, transphobia, xenophobia, or casteism”, as well as the sharing of “intentionally false or misleading information”. If you want, you can jump to another instance with looser rules, but be warned: admins can ban users, but they can also ban whole instances, “defederating” them. When rightwing social network Gab switched to the Mastodon protocol in 2019, it brought with it a million users, and was promptly defederated by almost every major instance, leaving the users in a bubble universe where they could talk to each other, but not interact with the wider social network. As Mastodon grows, those differences in norms are causing friction. Journa.host is an instance set up by former New Yorker staffer Adam Davidson that tries to offer a form of verification for the platform, allowing registrations only from journalists who prove they are who they say they are. But the instance has been defederated by almost 50 others, according to one tracker, with reasons given ranging from “privacy risk” to “mainstream propagandists” to simply “Journos”. The service is also suffering more prosaic technical pains as its user base grows by millions each week: larger instances are struggling to update posts in real time, admins are watching the moderation backlog grow, and costs are mounting for volunteer hosts who never expected to be absorbing an appreciable fraction of the traffic of a $44bn social network. But behind it all, the promise – of a ground-up approach to social media, where communities decide for themselves what they will and will not put up with, without thin-skinned billionaires putting themselves in charge at a whim – is enough to keep users sticking around. And eventually the jargon will start to make sense too.