Certainly something in your sidebar. Then, I know /r/askhistorians doesn't spam their podcast, but the mods are very quick to plug it when relevant, so keep an eye out for opportunities. Finally, just be sensible. Most people who are still on Reddit (I still lurk my niche communities) just want content. If there's a reasonable chance to tell them where else has content, some number will respond. I'd say y'all are fairly well equipped to draw in people if you think the Fediverse is THE FUTURE.