Reddit vs the Top-XX Sites

I've posted before how much I dislike those sites that ask you to link to them under the auspices of counting votes that move your banner ad towards the top of some sort of list. I call them "Top-XX List" sites. So when the following question was asked in the Private Forum, I decided that it really deserved a complete answer beyond, "because it works, that's why!"

I understand the arguments against Top-XX sites. I just don't understand how those arguments don't also apply to Reddit/Stumble/Buzz/etc.

The answer requires a discussion of social-networking sites and how the concept applies to sites like Reddit, StumbleUpon, Buzz, etc.

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