Effectively prosumers produce content for free, creating a sort of ‘digital commons’ - the owners of the platforms then use those commons to sell advertising space/ time, thus extracting surplus value from the prosumers.

I can't agree more. I think constantly about small things that we learn in life that we use to educate other sin our writing. Even content driven sites or clickbait articles use writers that source their information from your writing, your hard work, your unpaid labour, and then churn this out by spinning the words and gain the credit and the capital. It's a dog eat scholar world out there.