There's a Cat in the House and an
Elephant Mastodon in the room
What's Mastodon? Read: Welcome to Mastodon.
There's a list of instances. My recommendation: find the largest instance with solid uptime, currently accepting registrations. (There are information-theoretic reasons for this.)
More at Mashable, with a ... rather unfortunately hubristic headline: Bye, Twitter. All the cool kids are migrating to Mastodon..
I'll be interested to see how this scales out.