As per Nazna's figures, plus my own observations of each, I suspect Steam is currently somewhat ahead of 360 (not but much) and definately ahead of PS3.
I also suspect it's DD sales are better than either Live or PSN looking at the breadth of both AAA big titles plus smaller, indie titles it has to offer.
So yes, it has more customers - love the Freudian slip though!
Try to be reasonable... its easier than you think...