"nostr - Notes and Other Stuff Transmitted by Relays". "The simplest open protocol that is able to create a censorship-resistant global "social" network once and for all."
Jack stumbled upon it yesterday and created an account, so there have been a flood of new users already in the last 16 hours.
https://github.com/nostr-protocol/nostr
I recommend https://astral.ninja as a first time user as it's easy to generate a key pair, which you can then use as you try the many other clients.
This is a very new protocol, so the clients are still dev level and not production level, but development has been very rapid on this protocol.
One thing that stands out to me as a privacy advocate, is that unlike Twitter or Mastadon, the admins (relay operators in the case of nostr) can't read your DMs, so nostr is already ahead of both of these technologies in that regard.