Public rituals, rallies and ceremonies generate the necessary common knowledge. A public ritual is not just about the transmission of meaning from a central source to each member of an audience; it is also about letting audience members know what other audience members know.
This is an interesting idea. I would add that for at least some of rituals, I think an important aspect of their meaning is to make sure the participants in the ritual know that their society knows something. So, for example, if Bush were to find being President too stressful, the knowledge that everyone he knows saw him take the vow will add to his fortitude.
Or take marriage, another case discussed in the article. It is important that one's friends and family know one's status. That's the "simple" interpretation of marriage. Chwe would add that it is also important that the friends and family know each other know the status of the married couple. I can see some reasons for this, but it does not seem that important. It's more important that the couple know that their friends and family know, that they are married. In particular if one of the pair is tempted to cheat, it's likely that a potential partner will know his or her status, which will squelch the temptation in one way or another.
Generally speaking, a vow taken in absolute privacy is easy to give up. A vow taken publicly is not, because giving it up will probably mean loss of respect from those that know about it.
With superbowl ads, of course, my explanation is not operative. Except, perhaps, insofar as people know there is a huge commitment necessary to buy the time, so that to some extent having bought the ad insures that a company will stick with the service or good offered. (Psychologically true; economically such reasoning is false.) But Chwe's idea is helpful, I think, in analyzing why network effects might be the case in the mass media.
Postrel is always worth reading; I have her blog linked in the sidebar. That's where I found the link to the article.