As part of its settlement for a class action suit, Facebook will change its terms of service to enable, if not encourage, users to opt out of being used for “sponsored stories,” one of its potentially most effective ad formats. This may not be disastrous. Privacy advocates are way more concerned about privacy than is the average U.S. consumer. An economist for the plaintiff said these changes would affect only $100 million of sales (who knows how he came up with that). And it seems that the opt-out capability is case by case, rather than en masse. At the same time, comScore traffic data suggests that Facebook’s U.S. usage – as measured by monthly unique vistors – has plateaued. N.b., month-to-month traffic “trends” are no such thing. And comScore says Facebook’s time spent/user – the more important stat in the relatively mature U.S. market – is up. But these figures bear watching, especially if there’s even the slightest hint of harder-to-monetize mobile usage substitution. So far it’s been incremental to web usage.