While many ad buyers want guarantees that their ads get seen for X amount of time, Facebook has its own formula for what constitutes a valuable impression. It has to do with how fast people normally scroll on their phones and whether they hover on an ad for longer than their normal dwell time. It’s relative, according to Facebook’s Graham Mudd, director of ad product marketing, and even in less than three seconds, many users, particularly younger ones, are able to recall a message.