He clearly values privacy very highly, and using a social media network, even a free one means giving up some of your privacy. I doubt Stallman cares much about this, but there also is a very real effect of people competing with each other on social media to show more and more artificial happiness and consumerism in their lives. Thirdly, most of GNU development is still done via mailing list, if your using email for all your work, why not use it socially as well?
> using a social media network, even a free one means giving up some of your privacy
there's no fundamental difference between email and web-based messaging frameworks. a modern web-based messaging framework might even have better privacy guarding features than email, and more useful features by virtue of continued development.
Well the other feature of email in his case is all the GNU email list are archived, you can read his email announcing the GNU project still, I am skeptical any mastodon instance will still be running in 30 years. Mastodon as a whole may still exist, but instances come and go.
