Facebook Apps-types of apps that goes most viral

Hello Warrior,
I know quite well that social network is the way-to-go now and if you're not pitching your tent there, you're missing out! This is 2011 and we've got to move forward because there's an proverb which say

'when you see the bandwagon, the train has probably left the station'

Before the train actually goes too far, I think its gonna be a great idea to run and catch it!
Okay, I was just wondering,

'what type of facebook app do you think will go most viral?'

I am seriously thinking about it. I have this ideas
1. Game apps
2. Birthday remembrance apps
3. A social group within a social group i.e Arsenal fans app or chelsea fans app or man united fans app for soccer loving youth
4..........dump head....he can't think of more apps....go back to the thinking room or ask this gentle men to give you brilliant ideas!

The above are the ideas I have got. Can you make your suggestions in a list-like way like the one above? Let get the ideas rolling warriors!