They are, needless to say, not the most common formats. A search on google for "computing filetype:[X]", with "[X]" being replaced by pdf, epub, mobi, txt, and azw3 (prc produced seemingly no relevant results) has about 174 results for Kindle-supported files (done by going to the last page as Google's counter is more accurate there), 120 of which were .txt. The non-Kindle supported searches, epub and pdf, had 356 results, 67 of which were epub files. But Google's engine doesn't do great for finding books. A much better database is LibGen. As I couldn't figure out an analysis of all LibGen files, I searched for "Astronomy" (unless noted all LibGen searches are in nonfiction) which yielded 2,550 results, sorted by extension, and counted.
Results:
4 things in .zip format, 0.15 percent 3 in .rar, 0.11 percent 2144 in .pdf, 84 percent 11 in mobi, 0.43 percent 1 in mdf, lit, gz and bz2, 0.039 percent each 214 in epub, 8.39 percent. 156 in djvu, 6.1 percent 3 in azw3, 0.11 percent 8 in 7z, 0.31 percent 2 in chm, 0.078 percent.
In my experience that's rather skewed (I'd never seen a 7z one before, or at least not noticed one). But the general idea - PDF most common, then EPUB, then something else (often MOBI, distantly) - remains. The fact that Kindle can't go from 14 downloadable things to 228 (actually, many legal open access ebooks are published in just pdf or pdf+epub, it's rare to get mobi or azw3) by letting you download an already existent format it kind of supports is a bit ridiculous and has always struck me as odd.
But I am very curious how the color kindle works on the web browser, as noncolor Kindles have the background bleed a bit if you scroll. I imagine you'd end up with this sludgy mess of a background, but I'm not sure.