By the numbers: Do we have Facebook fatigue? - CNN.com: "The other problem with the possibility of hitting peak-Facebook? There's only one way to go from there and that's down. Users are finicky, and Facebook could find itself caught in a battle of just trying to retain users who might jump ship to Google's rumored new network or even just decide they are tired of maintaining a social networking profile."
Or as I have been thinking for some time, that the Internet web is really governed more like fashion, trends come and go. Microsoft is sort of the collector of trends converting those that last in to business tools.