My publishers don't give me stats that distinguish what ebook readers are purchasing my books, so I really don't know what percentage the kindle accounts for. However I also have a few Kindle books (ie exclusively Kindle) and they aren't exactly flying off the (virtual) shelves.
You can't tell us these things!
No, seriously, you can't. Term 7 of the KDP terms and conditions is:
7 Confidentiality. You will not, without our express, prior written permission: (a) issue any press release or make any other public disclosures regarding this Agreement or its terms; (b) disclose Amazon Confidential Information (as defined below) to any third party or to any employee other than an employee who needs to know the information; or (c) use Amazon Confidential Information for any purpose other than the performance of this Agreement. You may however disclose Amazon Confidential Information as required to comply with applicable law, provided you: (i) give us prior written notice sufficient to allow us to seek a protective order or other appropriate remedy; (ii) disclose only that Amazon Confidential Information as is required by applicable law; and (iii) use reasonable efforts to obtain confidential treatment for any Amazon Confidential Information so disclosed. "Amazon Confidential Information" means (1) any information regarding Amazon, its affiliates, and their businesses, including, without limitation information relating to our technology, customers, business plans, promotional and marketing activities, finances and other business affairs, (2) the nature, content and existence of any communications between you and us, and (3) any sales data relating to the sale of Digital Books or other information we provide or make available to you in connection with the Program. Amazon Confidential Information does not include information that (A) is or becomes publicly available without breach of this Agreement, (B) you can show by documentation to have been known to you at the time you receive it from us, (C) you receive from a third party who did not acquire or disclose such information by a wrongful or tortious act, or (D) you can show by documentation that you have independently developed without reference to any Amazon Confidential Information. Without limiting the survivability of any other provision of this Agreement, this Section 7 will survive three (3) years following the termination of this Agreement.
Note that section (3) indicates that all sales data is confidential and therefore you are not allowed to disclose it. You don't even seem to be allowed to say anything about Amazon, including 'I spoke to Amazon today about the misprint in my latest book.' Luckily, as the T&Cs themselves are publically available without having signed the T&Cs --- naturally enough --- it's possible to discuss them (see (A)).
I was intending to sign up for this, but the above clause seems unusually draconian to me.