Signal is probably going to be the best alternative that I can think of that has similar features to Snapchat for at least one on one chats and stories, but it doesn't have publicly discoverable content like Snapchat does
Idk much about Snapchat, just the disappearing messages part, which Signal has. That's FOSS, isn't it?
Pixelfed maybe? Depends on what part of those platforms you're most interested in.
For the social part of the Snapchat, it has a couple of features. Like locations, stories, streaks (which a few in my uni was too much into for some reason). And, let's be honest, Snapchat was used a lot for sending nudes back in the day, though I doubt OP was asking for that feature.
Subtle burn.
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