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:

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.