[durga]

It's somewhat hard to argue with a generalization like this. It's like saying: you give loans to 100 people meeting some very specific criteria, and the return rate is great. When you scale it to a million, the return rate isn't the same any more.

Of course the return rate won't be the same with 1 million people. But if you are grameen bank (http://www.grameen-info.org/), you institute mechanisms so keep the return rates high. That doesn't mean that there are no defaulters at grameen bank. It just means they were able to work creatively on a problem and actually meet their metrics (maybe sligtly relaxed) at very high scales.

It's non-trivial, but not impossible.

I think a measure of low quality is: do users/participants actually stop using it because of bad quality? It's a point of debate, but for me HN's quality is as good (probably better) that it was several years ago. Just my opinion.