George Clingerman:Whatever language you use has to be valid for the future as well. So whether it's true currently isn't your only concern. Definitely be vague. Look at existing marketing campaigns for things like Laundry detergent, etc. They specifically use words that legally protect them while still sounding like they just said they were THE BEST. I wouldn't personally make claims like "The Biggest Game on the Service" because someone could release something tomorrow much bigger and now you're a liar ;)
1. Get someone to write a review of your game that includes the phrase "The biggest game on the service" (or something to that effect)
2. Quote the review
3. Cite the date
Now, not only are you not a liar (if there's any incorrect information, it's the reviewer's fault), but assuming the reviewer has correct information, the statement is future-proof, since the statement is attached to a date :)