Facebook gets lots of resources: really smart engineers (worth much more than their salary) and tons of time/attention (that could have been spent on worthier pursuits than Farmville). It's not just cash.

(Your estimate of their cash impact is mostly correct, though: $2B in revenues for Facebook, http://schoolcenter.k12albemarle.org/education/components/la...; claims to serve ~12.000 kids in a county on ~$145M requested budget, and I could see a city having ten times as many kids.)