Two years after launching (in my living room at a TechCrunch event), web chat startup Meebo has begun to monetize their service.

Normal ad units don’t work on Meebo. While users stay on most sites for just a minute or two before leaving or creating a new page view by clicking on an internal link, the average user session at Meebo is 2.5 hours without any page refreshes. And 20% of Meebo user have sessions of 10+ hours – they basically never close the application. Selling ads based on page view doesn’t make sense at Meebo; instead, they had to invent a new kind of ad.

What they’ve done is create a persistent ad unit combination that allows users to click and add new buddy icons and background themes, watch videos, listen to music, etc. Or simply get rid of the ads. See the screen shot for a visual, and click for a larger view. Ads are charged a negotiated rate, at around a $10 “CPM.” In this case, Meebo will occasionally change the ads during a user session, up to five times per session. Each session is an impression, and a thousand of them are $10. If a user clicks to close an ad, no new one shows up in that session.

So far so good. The ads have only been up a short time and Meebo has just one sales person. Yet they’ve closed “tens of thousands of dollars” in business.

If the ads work, look for other sites to begin to look for ways to copy the idea. VideoEgg recently ported their popular Flash video advertising solution to make it

work on widgets in general, and Facebook widgets specifically. Stuff that works persists.

Hmm, I wonder how long it will take before people move to some other widget.