I'm not sure if the sharecropper analogy holds up. Sharecroppers essentially rent land, paying the landowner back with a share of the crops grown on it. Assuming the crop doesn't fail, both the cropper and the landowner make money.
On most web sites that aggregate user content, the content contributors make no money, and often the web site doesn't either (e.g. Wikipedia, YouTube, Facebook). These are more like online collaborative hobbies, where everyone involved is doing it just because they enjoy it and have some time to spare.
IIRC the rent was almost exactly equal to the value of the crops minus cost of living, so sharecroppers were left with no profit. Likewise in Web 2.0 there's usually enough money to keep the site running (sorry tr.im), but there's never enough money to pay any back to the users.
I think Atwood's point that many people are putting unpaid effort into sites that may benefit others is sound. I just don't think it's a calamity, since the time being put in is mostly spare time that the donors aren't expecting payment for.
On most web sites that aggregate user content, the content contributors make no money, and often the web site doesn't either (e.g. Wikipedia, YouTube, Facebook). These are more like online collaborative hobbies, where everyone involved is doing it just because they enjoy it and have some time to spare.