The key to getting people to let your expression-recognition software use their PC camera, is to make that into a service that delivers something they want.

Right off the top of my head, I am remembering a story that included an interactive CGI porn website that received some kind of feedback from subscribers and gradually mutated the characters and acts depicted to maximize their interest. In the story, the young naïve protagonist was cut off from the web for a couple of months (for the unwilling adventure central to the plot). When he got back online, he was completely repulsed by the current porn because he had missed out on the evolution.

But that just shows the problem with their technology: They didn't have individual feedback from each subscriber. If the facial-expression recognition company just offered free self-mutating CGI porn, they would get millions of voluntary subscribers. Then they could mine the expression data they received during the time subscribers were looking at non-porn webpages and market that to advertisers, etc.