This is a good example of how social filters work. A former student sent me the link to this via Twitter, and the professor for this course has made a very solid outline of issues that compare to our Digital Civ themes. Good bibliography, too.
This article briefly touches on many aspects of the Network Effect and the lock-in that can occur because of it. It is interesting that the communities that make some Web 2.0 services so successful are also the cause of their problems.