GetBig's traffic is the only traffic that matters.
Yes, tons more people visit bodybuilding.com, but they aren't bodybuilding people. They're there to talk about everything *but* bodybuilding, which is actually sorta cool for that particular community.
With respect to Muscle Mayhem, I'm actually quite sad to see what's happened to them in terms of board participation levels. I remember back when GB and MM were the only two relevant forums in our industry and suffice it to say, Mayhem was a GREAT site. But once they started going overboard with the censorship, within a matter of months, there was a noticeable decline in activity.
Just logged into MM for the first time since September and wow, it's practically a ghost town. That site was incredible back in 2003-2005, but censorship ruined a once-great forum. Oh, how I long for the good ol' days. *sigh*