eBay and Amazon were masters of transactional commerce, and this helped paved their way to rich market caps in the late 1990s. Yahoo! followed suit as the portal to the Web, garnering ad revenue. As the ad market changed, Google pioneered clicks better than anyone.
Whats next? Well, while many are quick to call for the demise of the pageview, the truth is that the pageview is still king with media planners and buyers. Pageviews, in fact, drive ad impressions and as F500 advertisers and global ad agencies spend more money online, they wont be giving up the impression metric.
Clearly, Facebook is becoming king of the impression, and user time spent on a site.
Lastly, it could be argued that the next holy trail in online advertising is average time spent on a site per visit. If that pans out to be true, then there is a lot of upside for Facebook.