(hah, fate has brought me back to this thread right after a question for me, weird!)
Tim, it's a mixed bag. Public posts are pretty much advertising, in the sense that I'm not looking to get anything out of the discussion part (if any). For WWWRPG in particular, though, I've had a small circle all through the development process of friends who have been interested in and giving me feedback on the game, and that's been super helpful. Most of the people in the circle ended up contributing to the game in the end (like Ian and Bret and Eppy). It could as easily have been a set of forum posts or an email chain but, in this particular case, it ended up happening on G+.
That said, one of the halo effects of writing a game in a subject matter that has it's own separate fandom is that you get to talk about the thing, even when we're not talking about my game. The G+ Community, in particular, has activity because people post stuff about wrestling in general and it's a shared interest community beyond just the game. But people are pitching ideas for the game and such, which is definitely inspirational to me on the creative level even if it's not an intense back-and-forth design discussion.
But yeah, posting from a position of honest enthusiasm and engaging with other people as fellow fans has generated the most activity for me over time, I think.
And for the record, the majority of my traffic for the Kickstarter (which is pretty much the only thing I have real numbers for) came from Twitter, not G+. In terms of actually getting the word out, Twitter continues to be a better venue than G+, though G+ is a more persistent medium.