What mode do most people play in L4D2?

I've been playing a little L4D2 lately on PC, but when I look at the lobbies there seems to only be a handful for campaigns or versus even though there are apparently tens of thousands of people online. Beyond that, I'm not sure what all those people do -- there aren't many campaigns in this game, so I am not sure what draws people to continue playing after all these years.

So I'm writing, what do most people choose to do when they boot up L4D2? Play through a campaign with ransoms? Versus? Something else that explains why I see so few lobbies?