RSS feed number from ongoing · How Many Percent?.
Interesting graph as I see a similar bobble in my own (much smaller) numbers. Could this be in part the effect of aggregators like bloglines?
I guess you could look for the number of incoming readers who did not fetch from the RSS feed. This does not tell you how many readers only visit the aggregator.
Update: If you really want (more) accurate numbers, you could always use a “webbug” 1x1 pixel image in all your posts. Presumably aggregators do not re-write image links and prefetch images. Somehow this seems just a little distasteful, given this was a favored technique of some highly (dis-)regarded advertisers. The image link would have to be different in every post, so as not to be cached by the browser. On the other hand – you didn’t hear this idea from me :).