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I'm not sure whether an iPhone-specific social network could take off, but iRovr is nonetheless having a try, and could inspire existing social networks to release usable iPhone interfaces.
The idea is very simple: blogging, photo sharing, video sharing, bookmarking and user profiles, all within the smaller dimensions of the iPhone screen. All new media appears in a stream of updates, and you can block out the media types you don't want to see. You don't even have to log in via the iPhone's browser: send emails to POP aliases to share your media.