Being on social media is very relevant due to both content discovery algorithms being able to connect people who may be interested in the project with the project itself and because social media sites can have things go viral outside of your own personal reach. Your post can reposted or spread by other accounts easier if its originating from the platform itself instead of hopping someone sees it and copies onto the platform.
Holing up in mailing lists definitely isn't going to help with pulling in users or devs.
[1] although it was maybe specifically just me they weren’t trying to attract.
[2] to the point where I actually worked with someone in my day job who was a debian dev and he wouldn’t sign my key without me producing physical official ID like a passport or something. Just really bizarre level of paranoia like a government kyc process or something.