No one claimed 60 million licenses mean 60 million users. But when this figure is comparable to the Win7 figures you DO have a very good indication of how well it's selling, because the Win7 figures had all the same variables.
I think Bas (below) makes an excellent point. There is one variable that differentiates W8 variables from W7; lots and lots of advertising. Either the advertising isn't working and thus a non-sequitur OR the advertising is working as a crutch to keep W8 even with W7 sales the latter which sold more on the reputations and merits of the product.