Measuring passive audiences for one media platform is difficult enough - what about developing a metric that tries to measure interactive engagement and involvement across multiple platforms. A number of major research firms are working on the problem, lead by Nielsen (looking to expand broadcast ratings to online) and comScore (looking to extend online metrics to broadcast and print).
report setting out some basic definitions and outlining six broad goals and 30 core metrics that should be incorporated into proposals for industry-acceptable interactive advertising measures.
The goals recognize that it may be difficult, if not impossible, to build one single effective measure - still, core metrics need to be comparable to those used for other media, and have achievable benchmarks of objective performance. The report also stresses that social media encompasses more than a single form of engagement.
Determining what you want to know is (or at least should be) one of the first steps in research, and particularly in the development of reasonable and valid quantitative measures. Too many of the traditional metrics for traditional media were based on what could be easily measured rather than trying to measure the things that those using the metrics really wanted to know. It's good that the industry is thinking about what it really wants to know about interactive advertising exposure and effectiveness, and isn't rushing to adopt something this time (despite Nielsen's several attempts to jump the gun and get the industry to support it's product).
Sources - IAB Redefines Ad Engagement, Clarifies Core Metrics Cross-Platform, Media Daily News
Defining and Measuring Digital Ad Engagement in a Cross-Platform World, IAB report