What makes Quora, Hashable, Plancast and the likes viral social apps?

If anything, social apps are games where your users provide the incremental AIs in the game in the form of inviting and engaging and luring other users into the app. This via invites, ecards, data import functions and whatever you can do to get existing users to entice new users into your app.

Learn too from the casinos and particularly the video-gaming gambling folks, and how they keep the engagement going with sounds and noises and colors and videos.

Learn from the multi-level marketing folks and from the secret societies, too. This via opening new levels or vendor coupons (from advertisers) or granting complimentary status levels and icons, this based on data imports or size of the network of contacts or whatever. And, of course, a way to buy this status is available, too.

Know too what your target demographic really wants. Gossip? Chats? Shiny? A source of information on new technology and on startups? Find it, feed it, and particularly work to get your clients to provide what your demographic wants.

And with games such as Farmville within social sites, we go full circle.