There exist many social calendaring systems out there such as upcoming.org, Renkoo, and to a lesser extent Meetup.com. All of these sites attempt to connect users via events. In a sense they're the most "social" of social networking sites, but for organizing simple, spontaneous social events they leave a lot to be desired.
Some friends and I were discussing ways to improve this over dinner a few weeks ago. To plan an event you generally need to know four things: who, what, where, and when. Upcoming requires that you know what, where, and when first. Renkoo requires that you know who and possibly what, but leaves the where and when to their fancy collaborative planning interface. Meetup is organized around groups, so you already know who, and everything else is left to a generic comment-based system.
In my experiency there are two kinds of social outings, what I'll call high-latency and low-latency. High-latency outings require a large degree of planning, coordination, and a certain amount of commitment. A conference is a good example of a high-latency social outing. Low-latency outings, however, require little planning and can be virtually spontaneous. A casual lunch with a friend you bump into on the street or a small gather of co-workers after the office closes for the night are good examples of low-latency outings.
The key to low-latency outings is that you rarely know who, precisely, but do know what and perhaps where and when. Imagine a more bazzar-like system, where people can post events ("I want to have dinner in China Town next week with at most five people") and others can jump on board. Once there are enough people, where "enough" is determined by the creator of the event, you could enter a planning stage or piggy-back off of one of the existing services, collaboratively working out the details of the event.
This approach works particularly well with people who already have some sort of social connection, e.g., alumni from the same school, co-workers, etc, since there is already a degree of trust among them. There is no need for a central planning authority: decision-making is decentralized. What's more, the group does not need to be geographically concentrated. Alumni from around the country can look for events posted by other alumni within X miles of their city, say.
Unless you're the President or a CEO most social interactions don't occur on a grand scale. If social networking is really about getting people together then it ought to be low-latency.
It also requires fun people. That would be my marketing strategy. Target people who want to have fun, or at least mock the concept to the point where people feel like they need to participate in order to have fun. Psychological mind games with the consumer.
This is very similar to a tool I helped build for PDX.rb — we call the app ‘Cat Herder’, and it’s designed to let anyone in the group propose an event and then make an open invitation for others to participate. I’ve wanted to rebuild it for a while now, since I think there are a large number of groups that have a similar need to casually toss out event proposals and collect RSVPs.