Amazon.com (AMZN 0.17%) recently bragged about the performance of its Kindle device family over the holiday shopping weekend. Between Black Friday and Cyber Monday, Kindle device sales more than doubled compared to last year. Cyber Monday alone was the "biggest day ever" for Kindle sales, which includes both e-readers and tablets. Unit sales were boosted by Amazon discounting its original Kindle Fire from $159 to $129. For some context, Amazon sold roughly 4 million units last December. Apple (AAPL -0.08%) recently launched its iPad Mini while Google's (GOOGL -0.20%) Nexus 7 is a worthy rival. Microsoft (MSFT 0.11%) recently launched its Surface tablet, as well. Unfortunately, Amazon, Google, and Microsoft don't disclose unit figures, much to the frustration of investors.