I sell the same number (approximately/usually) on a monthly basis to Apple customers as Sony customers. All my books go through Smashwords to reach either of them. I sell significantly more through Kobobooks and then again even more through B&N. From the authors I've talked to those going direct to Apple do have better sales than I do via Smash, but whether this is because they are more popular (The two authors I know do sell better overall than my books) or because they chose to upload directly, I don't know. I suspect it's just that their books are overall more popular so you can't compare apples and oranges.
There does seem to be quite a bit of buying via Kobobooks and B&N--both make efforts to sell books and have the infrastructure to do so.
Goodreads has a huge opportunity to sell ebooks, but doesn't have the infrastructure and so far hasn't pushed the opportunity. They'd need to sign on some publishers to get a bigger inventory to make a real dent, I think.
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