Yeah, so it sounds sketchy, and when you look at contracts, a lot of things can sound that way when it's in lawyer-speak, but as far as I know, they can't just make a show out of your work without your consent. Individual rights have to be purchased for everything, audio, visual, literary, etc. For each right they purchase, they'd have to specify a payment you can negotiate, and you can look these things up to make sure the percentage you're getting is appropriate. For publishing, they'd also have to specify payment for different editions (paperback, hardcover, special editions, etc.). They also usually give you a timeline for when the rights to any of these things will revert back to you and how much they are willing to pay to buy back the rights and for how long.
A contract taking all users rights just for using a site would not hold up in court, especially as there are most likely underage users/creators. Underage users approached for adaptations/publishing/etc. would be asked to have their parent/guardian sign the contract as well.
I'm not a lawyer, just have some experience with contracts.