ZOB is not the busiest center, real world. I'm pretty sure the Wikipedia numbers are either old are incorrect. Here's for 2010, least busy to busiest by flights handled.
HCF
ZUA
ZSU
ZAN
ZSE
ZLC
ZAB
ZOA
ZKC
ZDV
ZBW
ZMP
ZLA
ZFW
ZID
ZHU
ZME
ZOB
ZJX
ZMA
ZAU
ZDC
ZNY
ZTL
Here is from as far back as I can search (1989) to present:
HCF
ZSU
ZUA
ZAN
ZSE
ZLC
ZOA
ZAB
ZDV
ZBW
ZKC
ZHU
ZMP
ZLA
ZFW
ZME
ZJX
ZMA
ZID
ZNY
ZDC
ZTL
ZOB
ZAU
ZTL will be ahead of ZOB and ZAU before the end of this year.
As far as VATSIM, I don't know. I can't really find any useful way to crunch the numbers in the statistics anymore (which is a shame), outside of Vataware, and the statistics the Germans have, which is only valid since May 2010. As far as individual airports, it used to be EGLL, EDFF, and LAX. In 2007, ZLA had 77,000 movements just at their Class B's, which has to be up there. Unfortunately, the numbers are always going to be skewed because you can't check for overflights using any VATSIM data...at least I don't think you can.