So, how many people are using Twitter? According to Compete data, 14 million people visited twitter.com in March, a 76% increase from February and a whopping 14 times more than March last year. And this doesn't count the twitterati who rely on software apps like TweetDeck or Seesmic. The site already attracts more people than Ticketmaster, WSJ and LinkedIn, and the term "Twitter" had more queries than "American idol" and "IRS" across the top search engines last month. Like its social media predecessors, Twitter has captured the attention of consumers -- and marketers have to play catch-up once again.

Given this explosive growth, is there any doubt that brands will try to tap Twitter as a marketing tool? But therein lays the challenge: marketers haven't encountered anything like Twitter before. Despite its large user base, the underlying mechanics of Twitter are really about being atomically small. As a marketer, how can you hope to drive sales or create a branded experience when you're faced with a 140-character limit and a massively fragmented audience? How do you attract a following? How does it influence your other marketing programs? And how do you know if your efforts are creating ROI?

If you've read my other submissions to Metrics Insider on Moneyball Marketing, you'll know that I am a big fan of marketing that starts with a specific outcome, and then employs creative tactics and new metrics to measure progress and iterate along the way. And in this sense, Twitter is a great tool. While many people are content with Twitter's soft ROI impact (like seeding and/or listening to conversations about your brand), we are starting to see examples of Twitter's hard ROI impact, too. Southwest Airlines and JetBlue are tweeting promotional offers to followers, and Dell is probably the first company to achieve $1 million in sales exclusively through Twitter. And you can be sure these brands are taking what they learn from these campaigns and informing the rest of their marketing.

My personal favorite example comes from American Express, specifically its OPEN Forum. The OPEN Web site combines professional editorial, consumer-generated content and product marketing. To tap Twitter, American Express co-opted the popularity of its guest bloggers like Seth Godin, John Battelle and, in particular, Guy Kawasaki, (who wrote about Twitter on the OPEN Forum site just last week ).

Analyzing the traffic patterns of the OPEN Forum Web site reveals some great data. Every time Kawasaki posts a blog to the site, traffic doubles. Looking at daily traffic patterns since the beginning of the year, nearly every peak in visitation to the site corresponds to a post from Kawasaki. This is significant for a site that is already averaging over two hundred thousand visits each month.

So how is American Express promoting this great content - banner ads? SEM? SEO? None of the above. Instead, the brand relies on Kawasaki's existing Twitter celebrity, knowing that his tweets reach over 100,000 followers, who will then retweet the post to their followers. All of this activity drives traffic to the OPEN Forum Web site, where American Express can convert them into the right card product. One of the most telling statistics, however, is that Twitter is now the top-ranked traffic source for OPEN Forum - 11.7% of traffic to the site comes from Twitter versus 9.1% from Google and 4.5% from Guy's personal blog, GuyKawasaki.com. The data show that American Express is cracking the code on getting hard ROI from Twitter.

Twitter is to marketing what an atomic fireball is to candy -- seemingly harmless on the surface, and hugely transformative at its core. The trick is to start with an end goal in mind, whether it's clearing out inventory (Dell) or low-cost customer acquisition (American Express) -- and then tap into established and influential Twitter entities to help get your own program to scale. Sure it will involve trial and error, but with the right tactics and metrics, the ROI is there.