Discord is a likely contender, but I think it's likely to be Instagram. It's got a very dissatisfied userbase, and there's already a few reasonably active pixelfed servers
I'm not very familiar with Instagram either, but it's infamously bad for its users' mental health, it's recommendatiom algorithm is unreliable, and it collects a lot of data. Honestly it's possible that a lot of its regular users do like it and that I just don't tend to hear from them
As others have said, ads. The app is also overrun with spambots posting links to scams to buy followers or ForEx or whatever they're selling. The algorithm is really, really bad, at least if you're an artist of any kind. IG was one of the first apps to attempt to steal TikTok's short-form video format, which means it's hard to get your content out there if you're just posting images. And there are a LOT of content reposters.