Asking if using Twitter generates significant revenue is kind of like asking if RSS feeds or covering city council meetings generate significant revenue. The way I look at Twitter, or any social media, is this: Twitter alone, or even social media alone, may not generate significant amounts of direct revenue for a news org. However, they are valuable tools for serving your audience. And you use these tools not necessarily because they generate revenue, but because they serve the readers, just as council meetings may not be the thing that make papers fly off the rack, but you still cover them because it serves your readers to do so.
As for what social media gives back to the news org, I don't know how much direct revenue it produces, but I think there are a couple more important things a news org gets, which might indirectly lead to more revenue:
-- Expanded reach/influence/reputation: If you use social media tools well, it'll enhance your reputation as a reliable and useful source of information, a place that's among the first to pop into your audience's mind when they're thinking about where to go to find info on X. I think that's important in an age when news orgs are competing with millions upon millions of information sources for attention.
-- Additional resource for reporters: If you build a big enough social media community (and one that actually interacts with you), whether in the form of Facebook fans or Twitter followers or whatever new app that hasn't been cooked up yet, you get to a point where you can start using that community to help you do your work better. Need to find some sports fans with unique pregame rituals? If you have a few thousand or even just a few hundred followers, chances are a couple of them would know someone who fits the bill. Didn't make it to the site of breaking news in time for pics? See if any of your followers did. Need to compile a report of roads blocked by toppled trees after a storm? Ask your Facebook fans to post what they've seen around the city.
