NBC has been running ads on Facebook to promote the upcoming season of Chuck - airing tonight at 9pm. The ad use Facebook's event units: you can RSVP to 'attend' the event. That RSVP then appears in your feed and, if popular enough, becomes viral. This format is not new and seems to be popular with media advertisers (used to feature new releases such as movies, DVDs, etc).
But, you will see below that, depending on your RSVP, you get the option to become a Fan on their Facebook page as well. Here is the Chuck ad:
If you RSVP "Maybe" or "No", your calendar updates accordingly:
But if you RSVP "Yes", you get a confirmation spot that allows you to invite friends and become a fan. Its a small tweak but makes the units more effective and social