Reddit is a good one.
https://thehustle.co/reddit-fake-comments
https://www.forbes.com/sites/jaymcgregor/2017/02/20/reddit-i...
https://medium.com/@coinmall/how-easy-and-cheap-it-is-to-man...
https://www.cnet.com/news/reddit-election-misinformation-201...
https://mashable.com/2014/07/31/unidan-banned-reddit-permane...
https://www.youtube.com/watch?feature=youtu.be&v=YjLsFnQejP8...
https://www.youtube.com/watch?v=FxNvUWN3vYk
Also, reddit has changed comments before, or their CEO has. They have probably also banned people for less than transparent reasons.
Twitter is even worse with their shadow banning, and limiting viewership of twitter accounts, censorship of mainstream conservatives, while notably not censoring known terrorist accounts.
Youtube is known for allowing pedophilia posts, but having conservative politics also gets people banned.
I like mastodon, but I haven't used it enough yet to have a real opinion.
Also scuttblebutt (https://www.scuttlebutt.nz/) looks very promising but has made little real progress into mainstream as of yet.
Just a point of clarity - I am registered green party, but I don't think you can have true democracy with media manipulation.