I disagree here. Facebook did a few things incrementally better than its prior contemporaries. Remember when Friendster and MySpace had more users than Facebook? Facebook won social networking because it essentially fixed a few problems that plagued its competitors. MySpace was/is a mess. And Friendster was too self-serious (forcing you to post a personal photo instead of an avatar) and restrictive. And both MySpace and Friendster got privacy wrong from the get go. One of Facebook's earliest appeals was the privacy it allowed users.
Another factor was how exclusive it was - first Harvard only, then just US colleges. People were anxious to get an invite, for Facebook to open up globally. When it did happen, it was massive.
Creating exclusivity can create publicity in its own right and increase expectation. It can also fail though - look at Google Wave.
