In general terms, social systems group a bunch of people around a shared goal or interest. Further, in order for them to engage, you have to give them something relevant to engage around. Said another way, there must be some relevant collateral in trade among the members of the group as well as the means to conduct the exchange. All networks work this way whether it's the Boy Scouts of America, The Catholic church, Facebook, Q/A networks, or hybrid solutions like Twitch.
Example: Facebook is a status exchange network -- aka "what's up" social networking. Your "friends" are the group. Status is the thing in exchange. The timeline is the means of exchange and the focus of engagement.
Example: Houzz.com is a home remodeling and design network. The group consists of homeowners, design pro's, and merchants. The "images" (ideabooks and projects) are what's in exchange. The features are "add to ideabook" comment, share, built around the images. Participants engage around the images using the features Houzz built to enable it.
Every successful community/network follows the same pattern. Don't worry about things like gamification, or any other tactical social media-like behavior/best practice, until you've pieced together the basic exchange mechanism.
Therefore, ask yourself what is traded/exchanged in your network? What features have you developed that facilitate that exchange? One place to start is to ask yourself how the exchange happens without your service.