Well, since this is now bumped already...
I bet there's enough people playing FS2O that you could actually have people online at the same time, but it'd require that the game heavily facilitate that. For example, have a small notice box at the corner of the mainhall telling you whether there are any people online, and allow you to actually join them with 1-3 clicks (instead of going through the obnoxious '90's multiplayer interface). And have some servers/matches running all the time for people to join without the need to set one up yourself.
I'm pretty convinced that multiplayer is dead mainly because it's so inconvenient to use (compare to any modern game in which you'll get to shoot other people in a few clicks), and that would be possible to fix although of course it'd be a big undertaking.