It appears there have been recent complaints at the Google Groups discussion area that FeedBurner stats are way down for many folks.
Last night, Google, the owner of FeedBurner, confirmed that there is an issue with Bloglines tracking, saying:
We have seen many reports that Bloglines subscribers are not currently being tracked. We are making adjustments to restore Bloglines as a subscriber source and will update this issue once it's resolved. (Please note that any subscribers on Bloglines are still receiving your feed updates; they just aren't being reflected in your stats at this time.)
A Googler updated that document and said it has been an issue since about December 6th. Since Bloglines is a fairly popular reader, your FeedBurner stats may be way off.
Forum discussion at Google Groups.