After a year or so of LibriVoxing, I started defining (to myself) a successful audiobook as one that averaged 1,000 a month of downloads in its first year, for 12,000 total. Normally, there is a flurry following a new release, but demand decays as listeners favoring that type of book either listen to it or (if available) another version performed by someone else.
Given that kind of behavior, I decided 100,000 downloads was A Big Deal, and I was extremely gratified when two of my favorite books of all time - "The Mysterious Island" and "The Swiss Family Robinson" - both broke that barrier in April, 2008. Between then and this month, four more achieved that threshold: "Great Expectations (Jul, '10), "Adventures of Huckleberry Finn" (Sep, '10), "Tarzan" (Oct, '10), and "Adventures of Sherlock Holmes" (Jan, '12).
This month, two more joined the club! "How to Live on Twenty-Four Hours a Day" and "Around the World in Eighty Days" probably made the list the same day; I don't survey often enough to tell.
What's up next? I have one more now in the mid-90's that will probably "cross the bar" this year - but I'll wait for the event to tell you which!
(... And the ones that are clocking only 100/month, I doubt I'll mention at all!)