
Its just that now, we are not living in the more simplistic world of CPC, CPM and CPA style calculations. Engagement has multiple dimensions to it in terms of metric components. While every company must sit down and determine what they deem to be the proper measurement approach, here's one generic formula that can provide a starting point for this exercise.
Audience Engagement is a function of the number of clicks a visitor generates at a site, the amount of time they spent at the site, the frequency at which they return to the site, and their loyalty to the site as a member of the category for all of the sessions to that site during the reporting period. This online behavioral measurement should be looked at in combination with qualitative research data (such as surveys, polls and focus groups) to provide validation and deeper context to your reporting analysis exercises.
Σ (Ci + Di + Ri + Li + Bi + Fi + Ii)
The components of the calculation are:
Click Depth Index: Captures the contribution of page and event views
Duration Index: Captures the contribution of time spent on site
Recency Index: Captures the visitor’s “visit velocity”—the rate at which visitors return to the web site over time
Brand Index: Captures the apparent awareness of the visitor of the brand, site, or product(s)
Feedback Index: Captures qualitative information including propensity to solicit additional information or supply direct feedback
Interaction Index: Captures visitor interaction with content or functionality designed to increase level of Attention the visitor is paying to the brand, site, or product(s)
Loyalty Index: Captures the level of long-term interaction the visitor has with the brand, site, or product(s)
Using Comscore data, here is an example using news sites:

Using Audience Engagement (AE) we can say with a high level of certainty that a greater percentage of Internet users find CNN more engaging than MSNBC and Yahoo! News.
Final Thoughts
By establishing standard engagement metrics I believe that brand advertisers, advertising planners, and marketing managers will be able to use this data to make better decisions during the ad planning and media buying process. More importantly though, we can begin to look at the customers relationship with a brand in a more holistic way.