Just how many impressions are available for sale on the Web? The number proves staggering enough to approach infinity at this point, and there’s no sign it will shrink. As I learned on my first day of economics class, when supply soars, price plummets. The ubiquity of online ad space is driving down CPM (media planner speak for cost per thousand impressions) to a range of just a few dollars, sometimes pennies, for major ad networks and big buys.

However, several sites are bucking this trend and demanding premium prices for their roadblocks, banners, and videos. There's also that social media thing, which poses a new quandary: Can the popular content providers, such as celebrities, monetize the fact they have a huge following?

Of course they can. After the recent Twitter explosion, no shortage of companies are creating (or promising, whether they came to fruition or not) ways to advertise on Twitter, including TwitPub, Be a Magpie, Pay-Per-Tweet, and Super Chirp. This does not count Twitter’s own to-be-revealed solution. On this list posted at AdAge regarding sites and networks that charge premium CPMs, a slide was included, detailing the fact Kim Kardashian is paid $10,000 per Tweet,. This didn’t seem to surprise AdAge readers, but it may catch the eye of your normal Twitter user.

One service getting a lot of attention (some say controversy) over the past few weeks is Ad.ly, the service that set up Kardashian with that highly paid Carl’s Jr. tweets gig. Ad.ly, which launched in the fall, added analytics last week, making their self-service platform even more attractive to potential advertisers. Now, they can access valuable user segmentation and geographical data.

As for Kardashian’s recognition as an expensive tweeter, I’m sure it doesn’t hurt she is set to promote Carl’s Jr. through new television commercials as well (in the same vein as their promos starring Padma Lakshmi and Paris Hilton). While $10,000 per tweet may seem like a fortune, Kim’s vast Twitter following tops out at over 2.6 million, leading one astute commenter to point out that the CPM still doesn’t add up to what the other sites and networks ask their audiences for banner advertising.

Let’s Kim has exactly 2.6 million followers, and half see the Carl’s Jr. tweet. This means an audience of 1.3 million would give Kim’s tweet a CPM of less than $8. Sounds like a bargain, right?

It sure does -- especially compared to the roadblock prices for a day going up to $500,000 and CPMs in the hundreds of dollars for specialized or exclusive audiences. I doubt all of Kardashian's Tweeps are C-level execs. Other highly followed celebrities, such as Dr. Drew Pinsky, are careful about the offers they accept through Ad.ly and other services, wary of diminishing their own brand and credibility. The way that the online ad model and pricing structure are changing reflects how the Internet and content as a whole changes, contrary to previously accepted theories.

As more and more content comes from users as opposed to publishers, advertisers get caught trying to compare and contrast audience as well as content. The struggle for a balance or domination will continue in 2010. I can’t wait to see what Twitter comes up with for advertisers -- a lot of dollars are to be spent online -- and Twitter has the wow factor to rake in a lot of those dollars.