Their TOS says they own your content in any current or future formats or derivative works.
Their ToS could say they own you and your children and grandchildren, but that doesn't make it enforceable.
If I post a frame from the movie Akira on Reddit would any reasonable person suggest that they own not only that frame, but also the entire movie that it came from as a derivative work? There is a glut of second-hand data just like that all over Reddit, Twitter, and every other social media network, and I'm willing to bet that's also part of what's being sold.
But hey... I'm not saying you're wrong, just that the idea that they automatically "own" the things that people post on their website is ridiculous. It's a bit like UPS or FedEx saying they own the contents of your package while delivering it.