lol! Actually with ning you can sell your app since Network creators would love to add new and enhanced features on their networks, and willing to pay to get it.

there are 1000 networks being created everyday (based on what ning says) I would assume at least 900 are useless and would never come to anything, 75 would be small 7K+ users, 20 will be a good size 25K+, 5 will be 50K+. all these numbers are completely pulled out of my ass so please don't hold me to them it just to show you that there is a big potential there, at least I think there.

I own a network on ning(for fun) that has 2500 members and it has only been around for 5 months, the site caters to a specific type of profession (court reporting) and so far I have bought 2 apps that I installed on my network and I can assure you I would by more if there were around.

If a facebook app is done right you can make a ton of money. Incentivized offers are a huge money maker when creating facebook apps.