There are a lot of applications already on the iTunes store vying for a share of the audience's finger taps. Social networking has been bestowed with its own category, but there are no clear winners, no runaway killer apps -- yet. But there are a few worth mentioning as marketing lessons while companies try varied approaches to combining various iPhone hardware features.

Enhancing the existing experience by changing the context
 

You can find iPhone-specific versions of Facebook, MySpace, CareerBuilder, LinkedIn and even Xbox Live's store.

"Xbox Live Friends" is interesting as an extreme because it shows you who in your network is currently online, saving you the hassle of having to boot up the machine in order so see who's available to play with. That's it.

Writing about it here seems like a no-brainer: the content is already web-accessible, and one tap on the iPhone saves the user a few minutes of what can lead to deep brand frustration. It's one of those situations where it's tough to make a quantitative case for how much that frustration is worth in the long run. Ranking as the sixth most popular social networking application in the store, and having 130 reviews at the time of writing this article, hints to that greater, long-term brand value.

Facebook is fun but limited, and it takes advantage of the uniqueness of photo viewing on the iPhone by allowing the user to flick through friends' photo albums, pinch to zoom out, spread two fingers to zoom in. This iPhone app shows how dynamic the Facebook platform really is because just having the functions that work best on the phone (status, wall, friends, chat, inbox) make it worth keeping and using.