Despite getting attention as a healthier Twitter alternative, most Mastodon "federations" have remained small. Mastodon—which functions much like an open-source Twitter—is decentralized in that anyone can set up their own "federation," or server, but those servers can interact with each other.

Mastodon was launched as a decentralized, social justice friendly Twitter alternative that was free of Nazis and harassment. But two years later, the biggest Mastodon instance is now Gab, a far-right social media network known widely as a gathering space for white supremacists.

Gab—which has been tied to the suspect responsible for the Pittsburgh synagogue terror attack that killed 11 worshippers—announced on July 4 that it had switched its backend to run on Mastodon’s software, instantly making it the largest Mastodon user, with more than double the number of users as the next largest federation.

Because Gab is simply implementing Mastodon's open-source code, there's no functional way for Mastodon to shut down Gab. This, of course, was part of the appeal for Gab in the first place. In the past, Gab had lost its webhost GoDaddy and had been banned from accepting donations via PayPal.

“Gab is now unstoppable and can never again be taken down as a whole ever again,” Gab said to Motherboard in a series of email exchanges.

Mastodon released a statement denouncing Gab for using its tool and claimed it will do everything in its power to isolate the site. It also said some servers in its “Fediverse” were blocking Gab domains, meaning Gab can't interact with them.

“Mastodon is completely opposed to Gab’s project and philosophy, which seeks to monetize and platform racist content while hiding behind the banner of free speech,” it said in a statement posted on its website. “The Mastodon community does not approve of their attempt to hijack our infrastructure and has already taken steps to isolate Gab and keep hate speech off the fediverse.”