If you dig into IBM's original team (it was the integrator) you'll find Oracle, Esri and the Census Bureau. The team even won an award back in 2000 for its work the site. The most recent IBM contract for American FactFinder I found was in 2007 for $89.5 million (WRAL Techwire). The tech design doc is here (pdf). I don't how to reconcile the two different numbers - maybe IBM didn't use all the money from that contract? And, It's not clear to me which Esri tech is currently used on the site. It was ArcIMS, but I can't imagine that's still in there.
A new (now in beta) Census API may change how the public access and visualizes data in the coming years.
- The Atlantic via @timoreilly