I recently came up with the idea of a website which would track, and report on, where people, and their friends (across multiple social networks), are, in physical space, in real time. For example, you could post on your Twitter that you are at a particular party and how you feel about it. Then, your friends (on Twitter and Google Plus and Facebook and MySpace and wherever else) would see that and possibly come over to the same party. As they make their own posts, such as yours, their friends would see that this party now involves a number of people which they know, and then they might attend, and so on. I might work on this, but before I start, I want to know if such a web-based service already exists. I ask because I have a faint recollection of reading about something similar, recently, and perhaps even on HN. Thanks, in advance, for any replies.