These slides from a presentation on Open Source applications in archaeology are worth a look. They've included lots of screenshots, which is useful because it demonstrates that open source applications are becoming much more user-friendly.
Wired makes a compelling case for Twitter as a 'Social Sixth Sense':
Twitter and other constant-contact media create social proprioception. They give a group of people a sense of itself, making possible weird, fascinating feats of coordination.
In theory I just don't get Twitter but in practice I do read some long-running threads on various forums where people can post a quick rant about work, about their love life, or just add a random disclosure. If I know the people posting then I find those threads interesting. And I also love Facebook status updates for the same reason - they don't require a response but sometimes it's nice when they trigger one.