Yet, the debate continues on the most measured medium in history. Even though we offer far more accountability than any offline medium, publishers must still work to show advertisers that users are engaged with their online ads.

Earlier this month, the IAB challenged Nielsen//NetRatings and comScore to be more transparent in their research methodologies so that publishers can reconcile discrepancies between the two measurement companies and their own server logs.

Assessing a site's potential to deliver ad impressions by number of page views and the number of unique visitors is beginning to come into question under the weight of the advent of AJAX and the increased use of video.

Many have dropped the page view altogether as an audience metric. To fill the void, vendors are coming up with their own interpretations of engagement.

Viewpoint has something called the Engagement Index for rich media ads, which takes into account things like ad display time, clicks-to-run and interaction rate. E-mail vendor Lyris came up with its own Engagement Index weighing things like opens, CTRs, unsubscribes, forwards, and resulting transactions to come up with its metric.

Meanwhile, comScore recently announced that how often a user returns to the site is a good metric for judging user engagement. This "visits" metric, defined as the number of times a unique person accesses content (with breaks between accesses of at least 30 minutes) is a key component of user engagement, explained comScore.

I think there is something to the loyalty factor when considering comScore's results in how viewers perceive the use of any one particular site over their competitors.

The introduction of these new metrics based on "visits" provides an alternative for measuring user engagement that tells us how frequently visitors are actually returning to the site to view more content.

While each of the "visits" metrics offers a different measure of frequency, the "average visits per visitor" is the most illustrative of return visits per unique individual during the course of a month.

Used in concert with the "unique visitors" metric, this measure can help give a more comprehensive view of a site's performance, according to comScore. Additionally, I have found that tracking "daily users" helps to determine the overall loyalty factor, yet another important way to gauge a site's performance.

If engagement -- as defined by how often a user returns to a site -- has become the gold standard, then it is incumbent on us as publishers to foster loyalty for ourselves and our advertisers.

With the pending DoubleClick and Google deal, speculation is that Google will increase their activity around display advertising. Consider though that vertical content providers are much better suited to provide loyal users with relevant content.

The challenge and opportunity therefore, for vertical content publishers, is to combine engagement metrics with strong compelling content to those users who are engaged with the site. Another is to offer content to users anytime and anyplace, through the Web and on their mobile device, for instance.

At the end of the day, no matter what metric you use, content is what drives loyalty and engagement.

This commentary is insightful. I recommend it to others.