That's a diagram I created to illustrate Gartner's Technology Hype Cycles
I thought about that diagram when I read Shel Israel's post "Is Facebook starting to fade?" He points to some early warning signs, but admits he doesn't have a crystal ball. He does go on to say:
Personally, I think Facebook remains in the most promising position of any new company I know. Social Networks is the killer app of social media and Facebook is the most enjoyable place I have yet seen to meet people with whom I share common interests. But change is in an accelerated mode these days, and if Facebook does not pay attention, it could very easily find it's position more vulnerable than it appears to be today.