In the online advertising world, media is sold in a variety of ways, the most common being CPM, or cost per thousand impressions, or views of the ad (the M stands for mille, Latin for thousand). Our site for small business owners makes most of its money this way.

Many advertisers limit the number of times an ad can be displayed to a visitor to 3 or 4 per day. The thought is that if someone didn't click on their ad the first few times it was displayed, they don't want to display it again since it would be a wasted impression that they have to pay for.

In my experience, this is shortsighted. I was visiting ClickZ's web site earlier today and saw an ad I wanted to check out. But I was reading one of their articles which put me in the middle of a thought process and I wanted to click on a related link before I got distracted. When I went back to the original page, there was a new ad. I started clicking around ClickZ's site to find that ad to no avail. It was gone. I didn't respond as fast as the advertiser wanted me to.

In the print world, you don't have to worry about this. You can always flip back through a magazine to find an ad that caught your interest. But the online world is very different. Once you pass the page and the advertiser instructed the publisher not to display their ad to you again, you're out of luck.

Thankfully, email is different. If you have advertisers for your email newsletter, you typically sell the ad spot for a particular issue so if someone reads the newsletter again they will see the ad again. The same goes for an email marketing campaign: as long as the recipient doesn't delete the message, they can take action when the timing is right for them.

Maybe it's me. But I feel that advertisers who limit the number of ad views per reader are selling themselves short. Your thoughts?