A couple months back, I was talking to a fellow Nano-er and we speculated about the number of participants in Nanowrimo. Will the number of participants always increase every year? We basically figured that after a certain amount of time, the number of people in Nano would be at "saturation." This means that eventually the number of participants will reach an equilibrium where there is a core group that does Nanowrimo every year and at a certain rate, there is another group that decides not to do Nanowrimo again which is balanced by the group of newbies.
Chris Baty, the founder of Nanowrimo, believes that there will be 25,000 participants this year. Five years ago, it was just him and 21 of his friends. Quite a phenomenon. But also in some ways, I wish I had found out about it at the beginning.