Between Audible and Amazon directly they account for ~50% of audiobook sales[1].

[1] https://www.bloomberg.com/news/articles/2018-03-13/amazon-tu...

Not the parent commenter, nor do I have data, however, I can also provide my own anecdotal experience like yours.

Everyone I know who is a heavy reader of ebooks uses Kindle, and usually also has a Kindle e-reader (with the E-Ink screen). From my own experience the people I've met who primarily use iBooks are the more "casual" readers who read several books a year, whereas the Kindle users tend to read many more than that. The e-ink screen is critical for literary enthusiasts as it's the closest thing to a real book, but is digital.