That is a VERY simple assertion relating to something that is a bit more complicated. I think most people here should brush up on the idea of attachment networks, or organizational ecology. I'll give you the short version, the top two companies win where preferential attachment is what determines whether a user stays with you.
Of course there are exceptions. For instance, one could be providing information on a web site that would be a challenge to get elsewhere. The Wall Street Journal comes to mind. With these companies, attachment is irrelevant, if a user has a great need for the information s/he MUST go to the Wall Street Journal.
Most of the web 2.0 startups I have reviewed, and there have been many, are NECESSARILY based around preferential attachment. I understand this tendency. After all, if startup Y can implement the idea of startup X in less than 3 months time, then startup X will have an issue to deal with. So it is in the interest of startup X to make sure that users prefer them. Add to that the fact that most are banking on providing the user convenient access to his OWN data, and you have the makings of a good number of also rans.
I always ask, what happens if the user takes his data elsewhere?
"It'll never happen", is usually the answer.
I inquire as to the possibility of a recession and a concomitant slow down in internet advertising. Like all of the 25 year old mortgage brokers 4 years ago, most of the founders look at me like I just came from Mars. There is a new economy, get on board or get out of the way.
So I walk a little ways off...
And wait for the wreck, maybe I can get a good deal on parts.