While https://Mastodon.social https://Mastodon.social registrations are closed to keep the site up during the flood, it's all open-source and federated, so you

While https://Mastodon.social https://Mastodon.social registrations are closed to keep the site up during the flood, it's all open-source and federated, so you can equivalently join https://mastodon.club https://mastodon.club or https://mastodon.xyz https://mastodon.xyz instead, and still communicate perfectly well with mastodon.social users (or other GNU Social implementations).