As much as I’d like to believe that our RSS feed subscriber stats suddenly jumped from the 3,000 to 8,000 range, it looks like the real cause is an issue with the data FeedBlitz is sending to FeedBurner.
Notice the sudden jump, along with the massive slice of the pie chart going to FeedBlitz:
A support thread on this issue can be found here.
FeedBurner has offered their own email delivery of RSS feeds option for quite a while now. Maybe now is the time to make the switch?