Tell me you're joking. You are _joking,_ right?
When I was publishing on Smashwords, my users could download to whatever format they wanted, a wonderful feature. However, now that I'm stuck on lulu, I'm not distributing many of their epub files, so the majority of my sales are via Amazon's Kindle format.
You're right, epub is a popular format, mainly because it can be read by anyone, on any device. I'm debating forgoing using lulu on my next book--since my sales there have been marginal--and instead offering it to my readers directly, once again in their choice of formats. If I do that, I'll have a better idea of how many use epubs vs. the other formats.
In the last four years I've only sold 3,500 e-pubs as against 4 MOBI files I've been asked for.
That's because MOBI is an outdated and rarely used, older Kindle format. It's unpopular, because it doesn't work as well on most newer Amazon devices or in the newer Amazon reader apps, not because there are fewer Amazon readers!
All that said, before I switched from smashwords, the majority of my sales were via smashwords and NOT Amazon.
So even Amazon finally recognizes ePub. I'd expect one day that MOBI may joined the exalted ranks of LIT, PDB, and DOC in obscurity, to be converted to something useful by the likes of Calibre.
It's already joined those others, as Amazon now only marginally supports it (no upgrades). Unfortunately, they've never offered any MOBI to their newer format converters.
It's been years so I wonder if times have changed and .docx is the new standard?
Not for readers, but for communicating work--especially for those who used to work in office environments (i.e. most publishers and editors), it is, since that's how most submissions arrive.
For readers, as Awnlee pointed out, .docx is a pain because you have to pay a monthly fee simply to read it (though dropbox allows you to read them via their website for free).
Part of the issue is the major difficulties with conversions due to the .doc format having four sets of format codes for it, and also due to it being proprietary to Microsoft.
The first part is mitigated by switching to the newer .docx, and it's seen as a 'standard' now mainly because most word processors (including Google Docs) support exporting in that format, even if users don't own a copy of WORD or OFFICE. Unfortunately, most younger authors are moving away from word processors entirely, going with the newer generation of 'stripped down' writing tools, so it'll slowly be phased out over time.
Better to use opendoc (.odt, odf) and/or .txt for exchange.
Unfortunately, it's harder to edit or flag text in those formats. Text doesn't show formatting changes, or color code changes, and only a few apps allow you to edit .rtf in any form. As for OpenOffice/Opendoc, since it's not as widely used, it's still easier to export in .docx, rather than expecting everyone you communicate with to install a learn an entirely new product. Again, most word processors accept the format. The newer writing tools the 'younger kids' are all using, generally don't export to other apps (except as .docx).
As for .docx, when someone sends a .docx document to me, I reject it and request .odt or .rtf. I'll not pay the MS tax to open a MS bloat. My last version of Word is 5.1a for Mac OS-9, which I've retired.
I may be mistaken, but I'm pretty sure that OpenOffice and the other tools also open .docx files, and convert them to your local formats for you, so there's no need to purchase anything (though it's a pain-in-the-neck to convert from and to different formats).
Generally, I open edited documents in a separate window, correcting my source documents an item at a time, so it's not an issue which format a file is in, as I can do that from a WORD or OpenOffice document easily enough.