I did a search at Guinness's site for MMO
http://www.guinnessworldrecords.com/Search.aspx?q=mmo and none of the results were for City. Then again, I did the same search five minutes later and the six results it'd given me previously were no longer showing up. So maybe their search is borked.
The six MMO records it had listed were for "Most Popular Console MMO", "First Sci-Fi MMORPG", "Most Money Received for a Virtual World", "First Console Sports MMO", "Longest-Running Sci-Fi MMO", and "First MMO First-Person Shooter". None of those were for City of Heroes.
So before citing the "City's in Guinness" thing to the rest of the world, someone may wish to verify that City is actually in it. Unfortunately, a lot of what flies around on Facebook isn't truth, and it'd be awful embarrassing if we started spreading this as a talking point and it turned out to be false. :/