This bothers me. Super Smash Bros. Brawl had a ton of people playing online, and so did the Pokemon games...so why are they on the list? 
Are you sure about this, though? I won't say anything for sure, because I haven't actually seen any numbers, but let's put it this way: Halo 2 has had more players online than Brawl could ever dream of, and that game does no longer have online servers up and running. It saw them cut a few years back, if I recall correctly. I refuse to believe that Brawl has seen the same traffic. Brawl shipped about 12 million units world wide in its lifetime, while Halo 2 has the lower number of about 8 million. But the thing is, most players of Halo 2 went online, and it's one of the most played games on Xbox Live in history. While I don't have actual numbers that back this up, I do believe it's safe to say that the 12 million units of Brawl were not all taken online.
Brawl is the kind of game best played with friends in the same room. The Brawl online experience never really was flawless enough to catch on with a big audience the same way Halo 2 did (I know this sounds crazy to Nintendo fans, but just because you love one thing a whole lot, does not mean the rest of the world does as well. This is very important to keep in mind when we're talking about these kinds of stuff).
The argument of "everyone I know still plays Mario Kart" doesn't work, because those people are within your own personal little group, and you do not represent the world wide audience of video game players. I know no one has actually used that argument in this thread, but I feel it's relevant. I'd be surprised if more than six million players brought their copy of Brawl online. Also, remember that when I said the game shipped 12 million units, that does not mean it sold all those units. That's just the units that was ordered by vendors.
Right now I cannot for the life of me find any numbers related to the amount of players who ever played Halo 2, and Nintendo infamously almost never reveal information like this, so as of right now there's no way I can be absolutely 100% certain that what I am saying here is correct, but I also wouldn't bet against it. Regardless of how many people who might at one point have played Brawl online, I think it's reasonable to assume that those numbers have dwindled heavily in the past few years.