I'm not sure how these statistics were compiled, but this article at Curiouser and curiouser! breaks down the top 40 aggregators by mindshare, download size, and number of (users?).  Several popular aggregators appear more than once because they are tracked by version number.

Our friend Syndirella #2 in the mix, but NetNewsWire heads up the pack.  Go figure, a Mac Os X application is the leader by almost double!  Are the majority of bloggers Mac users?