Twitter, the popular micro-blogging service that asks users to update what they are doing 140 characters at a time, has received its share of press in the past year. From rounds of funding to being plagued by downtime, the service has had both ups and downs. One of the most constant criticisms of Twitter is its apparent lack of a business model.

Coming from a media background, my philosophy has always been "If you have an audience, you can figure out a way to make money." And Twitter has an audience. According to Hitwise, Twitter usage is continuing to see phenomenal growth and current estimates peg the number of users at more than 2 million.

With that kind of audience, Twitter is poised to make a boatload of money. How just depends on the business model that Twitter decides to follow. Here are 10 ways that Twitter could make money quickly.

1) Banner Advertising -- This may not be a creative audience-based money maker, but it works. Advertising is based on audience size, and with a large audience, Twitter should be able to crank out some money using traditional banner advertising. Twitter is already trying this experiment in Japan, and if it is successful, there is no reason that the company won’t roll banner advertising out to Twitter users everywhere.

2) Paid Posts -- Banners won’t reach the hundreds of thousands of users who don’t actually use the Twitter.com site, but who only interact with Twitter through third-party applications or SMS messaging. By allowing advertisers to pay to send a Twitter post that is distributed to all (or a percentage of) Twitter users, advertisers will be able to reach the audience in a relevant format. Posts would have to be tagged as sponsored, of course, and there would have to be limits on the number and frequency that they are sent to users, but this would be a fast way to reach the vast audience.

3) Sponsorships -- If Twitter decides that it doesn’t want to manage a large number of advertiser relationships (which can be a big pain), it could opt for a sponsorship model that would allow one advertiser to sponsor the entire site for a period of time. That sponsor would dominate the inventory -- the banner slots, the paid messages, etc. -- and this type of sponsorship would not come cheap. But many companies will leap at the chance to be affiliated with such a popular and cutting-edge service. And the first few companies to do so will likely get a ton of publicity, as well.

4) Charge for ad-free usage -- No doubt many people will protest if Twitter adds advertising, since the service has been ad-free for so long. For those people, Twitter could introduce a no-ad option that would require users who didn’t want to see any ads to pay to use the service. This is something that Twitter co-founder Evan Williams did when he started Blogger, and it could definitely work again here.

5) Tiered subscription model -- Many Twitter users have been clamoring for a paid version of the product for months, offering to pay to use Twitter if it just would stay up. Twitter will always need to maintain a free service level in order to appeal to the masses and to continue to grow its audience, but there is clearly an opportunity to charge users for enhanced capabilities. For example, there may be subscriptions that promise constant uptime, offer the ability to include photos, or provide the option for other types of posts. Or, what if Twitter simply charges users $1 each week that it doesn’t go down? That would give users what they want, and would give Twitter a financial incentive to increase uptime, as well.

6) Offer a monitoring package for businesses -- There has been no lack of publicity for the companies that are using Twitter effectively. Zappos and Comcast come to mind. But most companies haven’t even heard of Twitter, let alone started using it. Building a business-to-business interface that would allow companies to monitor their reputation on Twitter -- and begin to participate in the conversation -- should be relatively straightforward. And if Twitter charged companies to use the service, it could make money quickly. (Perhaps this is better as an idea for a new company based on the Twitter API, which brings us to the next idea . . .)