Yesterday I was working from home and noticed a stink outside - like they were muck spreading up the road. I noticed online at the same time from some of my Twitter buddy list that there was a similar smell 70 miles down the road in London. Then by lunch time, the press started to cover what became a story - that a foul smell was descending on all of the south east of England.
Now and again this happens - people start chatting about something on an open social network and it very soon makes it into the news. I'm pretty sure it wasn't a Twitter lurker who noticed the story, but sometimes it is. And you can see how, when you might be trying to create buzz as a PR person, how quickly things spread and the time it takes before mainstream media picks up.
For what it's worth, on Twitter specifically, a site called Tweetscan.com lets you track and subscribe to topics online.