Many successful facebook applications offer "low-pressure social interaction". I would, largely, refer to this as "low-content social interaction". Either way, there's a market for it.
Constantly track your "retention rate". Do this per cohort: "of the people who joined 1-7 December, how many are still users?". You must improve your retention rate early, because once people have been lost they won't come back.
The users-vs-time graph often describes a "shark fin". This is because growth stops when the network becomes saturated. To fix this, seed in multiple networks, increasing the maximum total reach. Also improve your retention rate.
One way to improve your retention rate is "recall" notifications. For example, receiving another "action" from an active user may bring someone back.
Pages should have visual appeal. Pulling in friends' avatars (alongside some kind of data - it doesn't really matter what) helps here.
As an alternative revenue source, tie in-game points to marketing offers (eg Lovefilm).
Check out the 'Nexus' facebook app. It shows connections between your friends, revealing groupings. It's fascinating.