These sites all put the viewer into the content. More importantly, they create an implicit call to action for the people to whom you send it: if they want to be in the content too, they need to come to the site.

So what’s the metric? We can track the number of “makes” — the number of versions of these videos that were created. We can (with more difficulty) track the number of times the versions are uploaded/viewed on video sharing sites. We can track the source when people come to the site to create a video… including if they clicked through from a video their friends sent them. But do any of these matter? If the goal of the campaign is “buy from the Gap” or “get a Sony Ericsson phone” or “see the movie,” does it matter?

Maybe… but it depends on how you’re monetizing the audience.