Yesterday, NBCU's EVP of Sales and Sales Marketing Seth Winter announced that all of this Sunday's Super Bowl on-air and online ad units are now sold out. At $4.4-$4.5 million for on-air spots, these are the most expensive Super Bowl ads ever. Winter also reiterated something he said a few weeks ago: he believes a 30-second Super Bowl ad really provides a $10 million value "because of all the incremental exposure that the creative receives on all the different social media platforms like YouTube or publicity."

The comment resonated with me, because way back in 2007, in "On the Road to the $10 Million Super Bowl Ad," I suggested that eventually Super Bowl would in fact reach this level (note that NBC isn't selling the ads for $10 million - yet. It's only saying that's their value).

Forbes did a nice analysis of Winter's $10 million estimate, using NBC's data. No surprise, the biggest contributor to the incremental $5.5 million of ad value is indeed the widespread online viewing of Super Bowl ads. That extra exposure is estimated to add another $3.2 million of value per ad.  The remaining incremental value is split between an estimate of attributable increased brand awareness, sales and stock price lift (all equal to over $2 million per ad) and attributable media coverage of the ads (around $480K).

There is absolutely no question that online viewing is the greatest thing that's ever happened to Super Bowl ads - contributing hugely to the game's perception as an annual advertising extravaganza. It's quaint to think back to the Super Bowl's pre-online video era, when - if you missed an ad during the game - you'd only see it if you happened to catch its replay during a subsequent program.

Now, numerous sites offer galleries of Super Bowl ads, so viewers can watch and re-watch (and re-watch!) the ads they love. All of this online viewing adds up - last year's Budweiser "Puppy Love" spot has generated nearly 56 million views on YouTube alone, while Volkswagen's iconic 2011 ad, "The Force" has garnered over 61 million views.

But even with the explosion of online viewing, Super Bowl ads still mostly miss out on what I've always thought of as their biggest opportunity: viewer engagement. Super Bowl ads are still pure entertainment, mostly meant to drive awareness and provoke emotion, but not action. Sure, there are the periodic URLs or hashtags in certain ads' closing frames, but it's still very rare to see a Super Bowl ad overtly invite the view to DO something.

In our current mobile and social-dominated world, this is very surprising. When virtually every viewer has a smartphone in their pocket while watching the game, wouldn't it make sense to prompt the viewer to do something to engage them deeper in the ad, or with the brand, or the product?

Of this Sunday's dozen or so ads that have already popped up online that I've found, there's only one true example of a call-to-engage so far, from Dove's Men+Care, which is actually a re-cut of an earlier ad. Following a 50-second montage of heart-tugging video clips, the ad asks viewers to "share how care makes you stronger, with #RealStrength." Dove is then selectively posting submissions on its web site. This is exactly what I'm suggesting other brands do as well: use great Super Bowl creative to drive engagement, which can then be showcased and further shared among its community.

With last summer's Ice Bucket Challenge showing that, even without a high profile Super Bowl ad, millions of people can be motivated to do something fun (and even socially beneficial), it's amazing to me more game advertisers aren't making engagement a focal point. With tools like Twitter's new 30-second native video feature, plus many others, it's easier than ever for viewers to get involved.

Going forward, there's little doubt that online viewing of Super Bowl ads will only become more popular, further driving up the ads' estimated $10 million value. But for even greater Super Bowl ad value to be unlocked, advertisers need to make viewer engagement a top priority.