I know it seems like it wouldn't scale merely because we haven't seen it happen, but I don't think it's economically impossible or even economically difficult.

I know it seems like it wouldn't scale merely because we haven't seen it happen, but I don't think it's economically impossible or even economically difficult. I don't think there's any reason to believe that your costs go up more quickly than your income from donations/ads. Instead, I think what happens is you just don't get many users in the first place because all their attention ends up acquired by people willing to operate non-sustainable businesses with the aim of extracting value in the future. If by some crazy fluke there weren't any highly motivated money-brains in the social media space, I'm absolutely certain a sustainable non-profit open source solution would exist in place of reddit and facebook, and it would eventually accumulate nearly as many users as those sites have. Maybe not as quickly, and maybe while throwing up donations banners at the same rate as Wikipedia, but it would happen. But you need to get impossibly lucky for the entrepreneurs to ignore "the space" when you have 100k users, and by 1 million users the scent of "alpha" has drawn so many of the biggest, meanest, smartest of the bunch that you need to be an absolute juggernaut of momentum and name recognition (like craigslist and Wikipedia) to have any hope of competeing.