FB: I gradually felt more bored with it, so I started to log in less frequently. I think I haven't logged in for at least five years, but I had already stopped using it regularly about a decade ago.
Twitter: As above, but the toxicity of the environment was what pushed me away.
YouTube: Newpipe. I only log in when I really, really really need to message someone on the platform or if I need to upload content (like once in a year or less).
Reddit: the API change, although I have logged in a couple times last year to ask things we don't have active communities for here. Niche stuff.
Deviantart: their Eclipse "upgrade", which trashed all the search system and the way art circulated.