TWO weeks ago, YouTube celebrated when the number of videos viewed daily on its site reached two billion, a milestone.

But it also used the occasion to express its envy of television’s continuing hold on viewers: “Although the average user spends 15 minutes a day on YouTube, that’s tiny compared to the five hours a day people spend watching TV,” the company observed on its blog. “Clearly, we need to give you more reason to watch more videos!”

YouTube, however, faces a huge obstacle: very short videos are unlikely to hold interest when watched in long sequences. It remains to be seen whether viewers will ever be interested in watching hours and hours of typically two-and-a-half-minute videos, even if produced professionally and well matched to individual tastes and moods.

The end of a program  whether it has lasted two minutes or two hours  invites consideration of doing something else. In YouTube’s case, of course, the end comes often. Jamie Davidson, a YouTube product manager, says that the 15 minutes of daily viewing by a user typically involves six videos, with the conclusion of each presenting “a decision point, and every decision point is an opportunity to leave.”