Within the past month [it's guestimated that over 1 million webmentions](https://snarfed.org/1-million-webmentions) have been sent on the internet. There's not really solid research on how many sites physically send and/or receive them, but as of [last June](https://www.youtube.com/watch?v=bkc5afFohmI&t=2s) the conservative number was in the range of about 5,000 and the number is increasing rapidly, particularly with platforms like micro.blog (5-10,000 accounts) now supporting them out of the box and CMSs like Drupal (via plugin) and Perch having added them since then. This also doesn't take into account an anecdotally sharp uptick of adoption on WordPress via plugin, most notably by several WordPress core developers.
Mastodon already supports most microformats for webmention to be implemented more easily. https://github.com/tootsuite/mastodon/issues/122 and https://github.com/tootsuite/mastodon/issues/965
There are already a number of open source tools that might also be bootstrapped for something like this as well: https://indieweb.org/Webmention#Services