I think that the percentage argument is not the only important thing to consider, there are other anti-competitive practices which (when practiced at a large enough scale) become a monopoly even if "only" 50% of people use the product (friendly reminder that 50% of internet users is more than a billion people -- much larger than the population of most countries).
My point is not that Google is doing something illegal (and thus arguing over the legal definition of a monopoly is not helpful -- just like arguing over the legal meaning of the US 1st Amendment is not helpful in discussions over the concept of free speech). I'm arguing that it is a monopoly in the ordinary meaning of the word, in that effectively everyone uses Google and Google exerts a massive amount of power over their users.
But for a pseudo-legal argument: Google participates in a form of product tying[1] by requiring you to create a Google account in order to access Google Groups (which are publicly-run mailing lists and an unrelated product to their account service). Now since you don't pay for Google accounts this isn't a strict violation of anti-trust laws, but it is very close in concept to the sort of thing anti-trust laws protect against.
