I have my problems with Facebook, but the one thing they do very well, and for me the most appealing aspect of the site, is their incorporation of third-party applications. There are many options to choose from and it is constantly expanding: quizzes, travel maps, feeds and much, much more.
This willingness to add more functionality, especially third-party apps, gives me confidence that they will not go the way of Friendster. Looking at the popularity of Facebook's Travel apps; their willingness to accept 3rd Party apps; noticing that Google's PicasaWeb is superior to what Facebook has yet done; knowing that Google is becoming an App superstore; it's not unthinkable that the day will come when Facebook and Google will combine forces. Or one will swallow the other.