(Not to be confused in a context of romantical entaglement, although...)
Been meaning to for awhile to write a little social networking application, UpTo (in want for a better name so far). But have had no spare time so far, but hope to at some point in the future.
The main idea is to arrange things to do. You do this by listing things you might want to do, perhaps by restricting time and date. This are general things over days, weeks, or always. E.g. I am always up for a beer on fridays, I don't mind playing football any day of the week after work. This is week I am keen for a concert, etc.
Then one of your friends thinks: "I think I want to play football on Sunday, who is up for it?". A quick check on upto.flurdy.com lists whom might be want to play. Integrate this with email, IM or SMS and hey pronto: you have a game of footy! Similar things can be done with nights out, trips etc.
Interface must be intuitive so that people easily can add what they would like to do, and also so that they do not need to often change this. Complicated or repetative tasks will kill any usage.
Also as with any social networking site, spreading/extending the friends circle is a key feature. No point telling only yourself what you would like to do.
The normal friends, friends of friends, and everyone networks will probably be used. But also the ability to choose a select subset of friends for certain events would be nice. E.g. Only Non work colleagues may know I am up for a beer during the workweek, or non of your female friends should know you really want to go to a Star Trek convention, or something similar.
Including an event scheduler would be handy, with the ability to organise and invite people to specific things you will be up to, E.g. that Oasis concert next week.
Time will tell if I get round to this project. Or, as usual find someone else has done the exact thing, but better. :)