taxi share

I wanted to circle back around on the Taxi Share idea that was discussed here a while back. If we could make something that worked, it would be really worthwhile, and also pretty damn cool.

I was thinking about how to do it, and it seems to me you'd just need a system that:

1. knows where you are or can make sense of you telling it where you are

2. knows who your friends are, and who their friends are, etc., outward x degrees

3. knows where those people are, if they've checked in lately

4. assumes you're using the system to share a taxi ride

And then lets the users take care of the rest.

Once you have this, you also have a generic framework for pairing any two co-located people around any common interest. So you could abstract this and apply it to all sorts of things. Would be very useful for facilitating a variety of sharing-type actions throughout cities.

This kind of seems like a generic, open version of foursquare. But I'm wondering - could you cobble it together using a variety of the standard tools, twitter, facebook, etc? Could you make it something that people could use without having to sign up for, because they've already signed up for all of those other things? E.g. "Direct message this service and we'll reply with someone who wants to share a taxi ride."

Any thoughts here? Anyone want to take this and run with it, or else tell me this is half baked? (Actually, it is half-baked and I'm counting on someone else to bake it fully for me...)