I kind of doubt that. How many "things" advertise as "*Regions* most popular X"? From soda-pop to chewing gum; headed to cars and so forth.
False Advertising would be if they promise you a pony with your game and then don't deliver it.
That actually almost happened in the beginning of the game with the Pre-order Joined Trill Race, actually. Since it was released to Fed side only at the time. It took alot of complaints and a couple months before it finally was given to the Klingon side..
Of course that should have been the writing on the wall for the KDF.. but that's an arguement for another thread.
Actually, City of Heroes is the only MMO that uses run-off from electrically based super-humans, and is therefore quite literally "superpowered". The only one, as far as I know, making it the most popular (and I guess technically least popular?).
I think what it boils down to is the phrase "most popular." That's very vague. It isn't a specific claim. It doesn't say they have the most subscribers, or generate the most revenue, or have the most concurrent users logged on.
I'm guessing that's why there's no fear of retribution with respect to truth-in-advertising laws.