It's not theirs. What you grant them is a non-revocable permanent worldwide license to use the content.

This is mostly necessary for the service to function, which is why it never really got pushback in the "early days" when communities were more tech literate. You need to be able to serve the content to users, and to a lesser extent being able to share popular active discussion topics is a big part of enabling the service to form communities.

What clearly isn't necessary is the "non-revocable" part. People should be able to delete their posts, and excluding for the purpose of moderation, have them removed. What also would be part of an "ethical" platform (to me), is a clear restriction in purpose to that license. I would limit my rights to the ability to use the content for the purpose of providing the "forum"(/whatever), moderation, and sharing public posts/comments to attract people to the community. But that's something that isn't trivial to write a contract for, and it is worth noting that unless they gave away DMs (which is extra awful), all of this content was deliberately public.

You could, as a host of an instance, have mostly whatever terms you want. The code is open source and it's not typical for open source licenses (including the GPL) to restrict things like that (you could probably structure a license that qualified as open source to prevent you from doing abusive things to end users of a service, but restricting how you serve it at all is unusual).