One of the consequences of Readmill shutting down is that I've lost track of my reading progress for any of the books I was reading along with my annotations. I am a victim of the ease of use of electronic books and book readers.
Readmill does offer a way to export reading data. But machine readable XML and JSON formatted data is useless unless that data can be imported somewhere.
These files are primarily for programmers. Export user info, followers, followings, readings, highlights, reviews, comments and likes on your highlights and reviews, and highlights you liked into JSON and XML files.
What the fuck are regular readers supposed to do with that shit? But really, what are programmers supposed to do with that shit?
The most useful file I exported is a CSV file with my reviews and reading history. I imported this data to my Goodreads profile. At least I can still read my books.