To add to this, can we "disown" what we post? If it can be used against me but isn't "mine", it just seems like I'm always getting the short end of the stick.
Generally you own what you post, but you have given a license to the site to display and use it. In the US, social media sites are protected by something called Section 230. This allows sites to display user-generated content without being liable for legal issues related to it, as long as they do a decent job of trying to moderate clearly illegal content.