[RobertKohr]

There is a set of knowledge in the world. It is vast, but much of it has already been contributed to wikipedia. The knowledge of the world continues to grow, but I don't think at the rate that it is contributed to wikipedia.

Contributors are really mining the collective knowledge of the world, and so they are running out. So similar to mining an nonrenewable resource, the cost of extraction goes up with time: http://desmond.imageshack.us/Himg521/scaled.php?server=521&#...

With difficulty in making significant contributions dropping off, you would expect to see a drop off of new users (they really don't get rewarded enough for their efforts). Back in the day, you could contribute something meaningful that you know to wikipedia. Most likely now, someone has already put it in.