Well, first I have very basic information about this social network. What is it about? The key thing I would recommend is finding your Unique Value Proposition and what do you have to offer your users that they can't get from any other app. Gamification is a good way to motivate users to share and for other things, like: to earn badges, prizes and so on. Also, having a good valuable social network makes it easier for your users to want to invite their friends to participate.