Hitwise has released data that suggests Bloglines is still the clear leader in the RSS Reader stakes, with 3 times more US visits than its nearest web-based competitor Rojo. I don't think a lot can be gleaned from this data though, as it misses out desktop-based RSS Readers, start pages and a number of other types of RSS Readers. Also as Mike Arrington pointed out, Google Reader appears to be under-represented in Hitwise's stats. Indeed Google Reader is currently not being counted in Feedburner's stats and I agree with Mike that it is likely to be around the same level as Bloglines at this point. The non-reporting is a problem at Google's end and I understand Feedburner is doing its best to get Google to fix it.
There's no doubt that Google Reader has improved a lot since its initial release - and is now at the forefront of innovation in browser-based Readers. Likewise I am fond of a few of the desktop readers, for example the Windows-based FeedDemon impressed me in my tests last year and the Mac-based NetNewsWire always gets rave reviews. Also a lot of people now use the start pages, like Netvibes and Pageflakes, as RSS Readers. Not to mention email subscribers and browser integration (Firefox Live Bookmarks, IE7, Flock, etc).
The bottom line is that Bloglines has much more competition - and the RSS Reader market has changed substantially - since e.g. Read/WriteWeb's analysis back in December 2004 of this market. So my hunch is that most early Bloglines subscribers (of which I was one) have now switched to another Reader - and perhaps even a different type of RSS Reader. I wish Bloglines had kept up with its early innovation, but sadly it hasn't and so now largely relies on its first mover advantage to keep the momentum going.
In any case, here's a quick poll which asks what type of RSS Reader you use currently - and feel free to back it up with comments.