From Summon Monsters? Open The Door? Heal? Or Die?, which discusses the declining value of blog comments and social news sites:
A funny thing about these sites is that they know about this problem. Hacker News is very concerned about not turning into the next Reddit; Reddit was created as a better Digg; and Digg's corporate mission statement is "at least we're not Slashdot." None of them seem to realize that the order from least to most horrible is identical to the order from youngest to oldest, or that every one of them was good once and isn't any longer. There's an entirely consistent and predictable theme here: devolution, and people who are surprised when it happens. It should be obvious, when it happens this consistently, that the decay isn't incidental; it's inherent. If it happens every time, for every site of this nature, it isn't coincidence.
Compare and contrast to Why are there no spam or trolls on Twitter?.