"For over the last 200 years, you've seen big companies grow off the concept of real-time updates," he says. "If you look at media companies like Thomson Reuters or Bloomberg, or the stock exchange, people and businesses are dependent on real-time news coming in as it happens. Twitter is similar, except it's much broader than just one kind of news, like stock updates. So when you think of it that broadly as a utility, I think you can begin to imagine how big the potential is for Twitter as a commercial entity."

The most obvious solution to how Twitter can make money would be to serve advertising directly in a user's Twitter feed (or "timeline" as the company calls it), or elsewhere on the site. But Stone and his fellow execs are wary of alienating Twitter's hardcore user base, which has grown accustomed to an ad-free service.

"How would they respond to us putting ads on the site?" Stone says. "Are we going to end up pissing them off?"

Stone says the key to making advertising acceptable to the Twitter community is ensuring that users choose which commercial messages they are exposed to, a lesson Facebook failed to heed last year when it was forced to quickly abandon its disastrous Beacon marketing system. But as with other potential revenue models under consideration, Stone would not elaborate on how he'd use such an opt-in system on Twitter.

One of Twitter's most likely revenue streams is through advertisements in search results where messages could be tied to what users were searching on. Twitter recently purchased Summize, a search engine specifically designed to sift through Twitter messages, for a reported $15 million in cash and stock.

"There is a pretty obvious opportunity there," Stone says. "There is a level of intent that someone is showing when they come to a Twitter search and type in, say, 'iPhone.'"

Another possible revenue stream is corporations paying to use the service to stay in frequent contact with their customers. Several large companies, including Dell, Whole Foods, and JetBlue, have already set up corporate presences on Twitter to let customers know about special offers and even answer customer questions.

In Dell's case, the company says it's made "well over" $500,000 in sales from sending special offers from its Dell Outlet store to its Twitter group, which it began in June 2007. The group has almost 1,500 "followers" who receive its messages on a regular basis.

"This is where are our customers are going," says Bob Pearson, Dell's vice president for communities and conversations. "These folks are influencers, and they want to talk about the hottest or latest stuff going on."

Dell also answers individual user questions via its Twitter group, which is what companies like JetBlue (almost 3,500 followers) and Whole Foods (2,000-plus followers) primarily use Twitter for.

To a company like Dell, $500,000 is a relatively small amount, but it does hint at the potential windfall companies can reap from Twitter. "If you've got 1,000 people following you, something's happening," Pearson says. "The real potential is in the future when you've got 10,000 people following."

While Stone says charging corporate users for a Twitter presence is something he might consider doing, it's not at all clear that companies would be willing to pony up for it. When asked whether Dell would consider paying Twitter for, say, each user that signed up to receive its feed, Pearson says "probably not." Whole Foods and JetBlue both say it's too early to say whether they'd be willing to pay Twitter for its service.

"One of the beauties of Twitter is that anyone can go on, and it doesn't cost anything," says Pearson. "There are other ways that Twitter can monetize its site, through advertising or other means. They don't have to be charging business customers to be part of it. But that's for them to think through."