Well, for example if I could reply to a mastodon post from my lemmy account - the poster would see that there (not here - but could show up on my profile page), and might follow it, so it could gain followers. To write such a reply, I’d need to somehow view the original post while logged into lemmy. My comments here do federate to mastodon, and if somebody searched for related words (at least from the instance from which I followed my #lemmy account) they should find this. Your “virtual community” seems like a mastodon list (I have a dozen such topic lists, that system could be better, but is improving), indeed it would be helpful to consider that alongside a lemmy community for similar topic.