I know several people who had this happen to them — and those are just the ones who've opened up about it, which obviously not everybody would do, so I may or may not know even more than I think.
It's far, far, more likely that you've had friends who drank too much and then assumed, in the absence of an actual toxicology report or other corroborating evidence, that they were drugged.
Ref: http://www.britac.ac.uk/funding/case-studies/burgess-spiking...