In the past I have had access to a number of communities web logs with traffic in the 1 to 25 million hits / month. After looking at the logs then I came to the same conclusion, the top sites have the same small group of active users. Two of them where competitors with all most the same offerings. Between the two competitors there was around 30000 core users.
When the traffic was compared to other very unrelated sites with similar volume traffic there was limited shared users.
The end result is if you try to copy youtube, you will have the same core users. The same result if you make a site like digg which attracts the same kinda user which would use youtube. Making yet another techie web site will not yield a new audience.
The conclusion I came to a number of years ago by the raw data is copies of sites don't attract more Mainstreet USA let alone more users.
