As much as I agree with the overall premise (Facebook won't be king of the mountain forever), I'm still a bit unsure about what the driving force will be for users to go from Facebook to "open" alternatives. The driving force for leaving AOL was clear: there's a big, exciting internet out there that's more interesting and innovative than AOL chatrooms and AOL keywords, and the user base was also getting a little more…
If social is a frontier with similarly unrealized potential, an open platform will eventually emerge and innovation will happen there. Cool new things that Facebook can't wrap their head around will draw people away.
One of those new things will become so popular and so essential that its centralized control becomes a general threat, and all this happens again.