Nir Eyal Thank you! I looked through the studies I could find and their citations, but most of them focused on digital vs. paper, which largely concluded that while digital was more available, paper was often a preferred option. In terms of adoption, the best I could get was an approximation from someone who’d done their own research, see the question comment/details section on this Quora answer.

One of the more interesting studies was this one, found here. It only states people use paper calendars on top of digital ones, though.

Our findings are similar: despite most of our users using electronic calendars, every one of them reported using paper calendars even if not regularly; 12 out of 16 interview participants reported using them regularly.

This study from 2011 gives paper calendars a slight edge over digital ones, but it might be outdated.