"The combination of "seems impossible" and "strong network effects" is about as close as you can get to the magic formula for incredible, sustainable success, as with eBay, Wikipedia, and Google."
Hal Varian says that Google doesn't have strong network effects working for it, and he might know a thing or two about it:
http://googleblog.blogspot.com/2008/02/our-secret-sauce.html
eBay certainly does, though.
Further, Varian's argument about knitting websites is a little off, since Adwords/Adsense effectively allows you to reach those customers via Google.
"The combination of "seems impossible" and "strong network effects" is about as close as you can get to the magic formula for incredible, sustainable success, as with eBay, Wikipedia, and Google."