Well, I'm doing my best to get the !openstreetmap@lemmy.ml community started. Once there is enough activity here, the critical mass becomes self-reinforcing and it takes off.
And it's not as if all of Reddit will switch over right away - Twitter is still somewhat alive too; but Mastodon gained enough momentum now to actually matter and to be at a sustainable level.
Look for a local hackerspace, they might have the right connections. Most of them have regular (often weekly) 'social nights' where you can hang out.
Https://Mapcomplete.osm.be/hackerspaces has an (incomplete) listing