When you post something on someone else's website then you give it to them. You never had ownership.
This is not true. You may retain ownership of your content. This means that you could remove it at any time etc. If the service retains ownership, then they could redistribute your content without your permission. Example are Reddit vs Facebook, whereby facebook retains the right to redistribute your content no matter what and Reddit always allows you to delete your content.
GDPR is about personal data not what you post online.
Posts are still personal data. For example GDPR allows you to request all your data and data they hold about you from a website owner. This includes your posts.