I think you are wrong about that. Social networking sites have a huge amount of "stickyness" because of the network. In the end, the value of a social networking site to the user is the size of the network. Social Networks are tricky things, there is no way that me and all of my facebook friends will collectively decide that facebook isn't doing it anymore and we'll move to twitter. For example, I signed up for twitter cause the concept and feature set seemed cool, but I never went back after more than two times because I only had two friends on it.
On the other hand, I think maybe one example of a social networking app falling is AOL Instant Messenger. AIM used to be the way everyone I knew IMed. around 1998 it exploded. However, in the past two years I have noticed that more and more of my friends are depending on GChat, and aren't signing on to IM anymore. About 25% of my friends now have abandoned AIM and moved onto GChat. I think the two reasons this happened are 1) the horribly bloated AIM software that is just unpleasent to use. 2) GChat sort of snuck in as an automatically activated feature of GMail and people started seeing their friends just showing up on their list. Remarkably though, AIM still, after 9 years, has three quarters of my IM contacts.