As mobiles, and the people who carry them, finally start to become part of the Internet I expect that one of the first killer apps will be mobile social networking. Unfortunately, today, it's still waiting for takeoff.
Social networking on the web has reached mainstream. I heard stats today that 40% of UK singles use online dating (and that's probably 90% of all singles with internet access then); 10% of newly weds in the USA in 2004 met on the Internet; and a UK divorce counselling service said that "Internet adultery" accounted for fully half of all reasons for divorce in their survey.
However, people have to switch off their digital prosthetic and go out into the rain and the real world to make do. Their urbane banter on IM or Orkut gives way to the rather more difficult task of meeting and speaking with real people. This is a pain point for many, and one that a mobile version of social networking tools could get right. The missing link is location information, and the operators are rather protective about giving up this information for other people to make money with. There is progress, especially in the US, which has implemented the E911 rule.
I predict that the transition path to a breakout success will not be a mobile startup with a great technology reaching critical mass, but will be an existing group of people deciding to integrate with a pre-existing mobile technology. I think aSmallWorld would be a great candidate, since there is a high homogenity across members - most members are happy to meet others, as it's still got an early adopter cachet; and there is respect for top-down proposals. These will make it easier to reach critical mass of adoption - always the problem with new networking services.