I've seen other book communities (of course, given my luck, I cannot recall their names at the moment) where the moderators post chapter discussion posts and they seem to do pretty well.

One thing that appears to help break the ice is if the moderator posts their own personal reactions to the chapter, and their own ideas about symbolism and what they thought about certain quotes, etc. Being the first person to come out and start a discussion is daunting. Having the mod get the ball rolling in such a way appears to help because people can respond to what you say and then form their own ideas to add to the discussion.

Asking specific questions in the post also stimulates the conversation. E.g., "What do you think of character A's reaction to character B's betrayal?" When there is something specific to respond to it seems, in my experience, that people are more apt to comment because it isn't this big general bubble that they have to react to. You could, if you don't feel like posting your own general reactions to the chapter, post a series of questions about the chapter to help guide members in the discussion.

However, like others said, when it comes down to it you might want to try out different methods and then stick with the one the produces the best outcome in terms of quantity and quality. (Blah blah blah, I swear, I'm not normally such a blabby know-it-all.)