purely ad-based revenue models are sustainable provided you can sustain the traffic. traffic, however, is often fickle, especially in the social realm, which is why it is usually "better" to get paying users. however, its a double-edged sword, as most social apps aren't worth paying for.

i highly doubt that social media will be viewed as a fad. it has already had too much influence to be considered meaningless in a historical perspective. imo, that is. it might not always look the same, but the concept will likely be around for a while.