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Sup is a New Messaging App by the Creator of Pixelfed
(wedistribute.org)
A community dedicated to fediverse news and discussion.
Fediverse is a portmanteau of "federation" and "universe".
Getting started on Fediverse;
I think it's intended to be more like WhatsApp, in the sense that you use it for one-on-one chats or messaging small friend groups. I don't think it's a current goal to try to take on Matrix / Element.
Those are exactly what I use Matrix for.
I think it is to just replicate Instagram DMs.