Apparently I can't stop singing the praises of Friendfeed. The thing about it, and perhaps there are other lifestreaming / microblogging services with this same feature, but the beauty of it is that it combines synchronous and asynchronous modalities so seamlessly. Like a busy Craigslist forum, IRC, or live chat room, the conversation sometimes zips along at a furious pace in real time. But unlike CL, IRC or live chat, someone else can easily stumble upon that same conversation two hours later, or three days later, or four months later, and have something to add to it.
IRC and live chat rooms also sometimes move so fast that the conversation is fleeting and not captured for posterity (except by savvy users who may capture the stream for various reasons of their own, but rarely share it). Friendfeed, on the other hand, is creating an archive that lasts. You can have your asynchronous cake stashed away and eat your synchronous cake too.