the best repeated advice I've seen so far is "build something that entices one or more cardinal sins". If your game/tool/app tickles jealousness, pride, gluttony, greed or any other major sin, you're in for a good start

4sq/gowalla work on vanity (people like doing things/going places to impress/make their friends envy) as well as gluttony (the 'badges' concept - eat all you can eat), and to a point, even sloth ('whatever, I'm _here_ and too lazy to call every one of my friends individually')