The 40,000 drivers came from the same Ubertweet as the 3.5M users.
The 3.5M users would be rather more than half of Londoners who have both a credit card and a smartphone. It is wholly incredible, if they mean any sensible definition of "user" (might be "has called Uber twice"). I suspect they're adding everyone who ever downloaded the app plus all the people in every cab, then multiplying by two.
40,000 drivers? I doubt it. Halve it, then assume most only ever worked a few hours.