In fact, I think this is one of the reasons so many people are trying to re-invent Facebook. If Google could re-invent Excite and Facebook could re-invent MySpace (and Reddit could re-invent Digg) then why can't someone re-invent Facebook? And maybe someone can. (I sure hope so!)
I think the difference with Facebook is the network effect and that it is run by Zuckerberg. What ever failings Mark has a CEO he has one amazing strength which is a single minded focus to let no competitor to Facebook rise. As he has shown time and time again he will buy you out or outspend if you have any possibility of being a competitor.
Yes, that was exactly the point I was trying to make: Google is a counter-example to the theory that you should not fight the last war. (On the other hand, it is also a positive example, because no one has been able to displace Google despite there being no shortage of attempts.)
In the world of business, this is well illustrated by the market dominance of SAP. They have, and will buy out, or compete out startups that encroach on their market. This is why they are still the top pick for the biggest companies in the world.
Successfactor comes to mind as an example.