KILKENNY -- In my earlier life as a command post duty officer, we occasionally had to notify a wide swath of agencies about an event. The highest level of message traffic was FLASH. Howard Rheingold inherits that idea with "Flash Mobs," and his Smart Mobs web site occasionally points to public gatherings of complete strangers, organised via blogs or mobile phones. When Flash Mobs do Dublin, they do stupid things like swarm into a shop, mill around and quickly disperse.
I think it's more worthwhile to organise technology around real meetings, so blogs that connect to MeetUp win my vote. And "speed dating" (perhaps organised with AnotherFriend's boards) is another recent phenomenon that occurs when people have a series of short conversations with potential partners in order to determine whether there is mutual interest.
Howard Rheingold writes SmartMobs the blog and the book. He blogs about Flash Mobs in Vienna.