Yet another step in the direction of tight realworld-internet integration: the number of startups proposing cell-phone based location aware software is skyrocketing. We've already discussed linkedin going mobile and the current problems of actually using cellphones to do social networking, but now the market is getting crowded.
The most straightforward use of such a network, and probably the one with the best ROI as of today, is the "Mobile dating" slice. MeetMoi and limejuice are doing it right now, but more will surely join in the future.
While some other startups are going in a "one network fits all" approach, like MobiLuck, Imity,Loopt), there is space for more specialized networks.
Think about a gaming platform, think about hobbists who seldom meet each other and so on. While a clone of facebook would likely result in a huge mess in any city as soon as it reaches critical mass, a focused application only connecting some kind of people could do the job: meeting even one new person into the "70s-singers-wearing-only-black-shirts-from-Losanna" fanclub could easily
Meanwhile the first IPhone powered social network is almost ready.