Amazon's "Kindle Select" lending program requires authors to offer their ebooks exclusively through Amazon to receive benefits of the program, such as being in the Kindle lending library and offering free ebooks once every three months. That exclusivity comes with a price, however:

"Almost like clockwork at KindleBoards, it seems like every other month or so, there's a thread of authors decrying their drop in sales at Amazon, and then the speculation ensues about why their sales dried up. Did Amazon change their algorithms? Maybe, maybe not. It doesn't really matter. Algorithm changes or not, there's a lot of randomness in this game at every retailer. Sales go up, sales go down, often without reason. When someone's getting 90 or 100% of their sales from a single retailer, and that retailer rolls over in their sleep and accidentally crushes them, there's not a lot of recourse."

— Smashwords' Mark Coker on the problems with ebook exclusivity

(via A Newbie's Guide to Publishing)