It's interesting how the Ecosystem works, by counting not just general permalinks, but also the more transitory links to specific articles that come and go. That's how, by taking a week off for vacation, Kevin Drum managed to fall from number five (I think) to number ten. So, not only did Kevin not produce anything new in that period to maintain his incoming article links, but as other bloggers kept on going, their earlier links to his articles in turn fell off their main pages.
(I don't think, though, that the Ecosystem will count different links to the same post on another blog, from the same blog. So, if Aaron were to link the same post of mine twice, but in two different posts, I don't think that would count as two incoming links, for Ecosystem purposes. But I may be wrong here.)