Sci-fi club would be awesome! I always find myself lurking stuff like that, even if I don't read enough sci-fi to contribute. I agree that we need more people - it's tough to know how to find them, and how to find quality users. What's caused our inflows in the past?
Reddit events, generally. Whenever there’s a flare up of “rah rah reddit is terrible” in response to some policy change etc., and in the event that someone mentions Hubski somewhere, there’s a small surge of users. Although sometimes people mad at Reddit because they banned r/fatpeoplehate are poor Hubskians.
I think this is one of the biggest problems - if you're leaving Reddit now and didn't leave reddit years ago you have no expectation of thoughtful discussion. The site is something completely different now, and the emigrants that came here are generations removed from what drives Reddit now.
Even back in the day though, who was Hubski really snapping up from Reddit? Probably disgruntled tech employees who hated its guts but still couldn't avoid clicking on it until it finally became local news-level entertainment. The only way Hubski is going to get more users is if Hacker News goes out of business or somebody takes their entire grad school hostage, whichever comes first.