The Facebook application platform didn't do anything particularly new. Take Scrabulous, a top 10 Facebook app. Games associated with friends lists are far from new; AOL, ICQ, and others all tried variants of this idea in Bubble 1.0, but none of them were able to bring IM to be more than just messaging. Even without integration, sending a friend a link to Yahoo Games wasn't a barrier preventing consumers from playing online games.
Importance of platforms is nothing new. IM took users from email, mp3 took users from CDs, and so on. What you're doing matters as much as where you're doing it, and certain platforms do a better job facilitating tasks than others. This isn't to say Facebook is the alpha-platform. Facebook may be moving into the IM market, they'll undoubtedly see stiff competition from existing services, as well as web services, and while their entry into the email market didn't go unnoticed, "Messages" hardly displaced email.
Presentation is as important as function. Not only are numerous startups, e.g. Meebo, based entirely on presentation, but superior presentation can quickly displace even a 5-year lead in time to market.