There has to be, because one weekend, the screen came up with a dialogue box saying that the WFC server was down... and it wouldn't let me even access the menues for Multi-player and Escalation. The only thing I could access for several hours was the single player Campaign part of the game.
My thinking is that people are just hosting the games on the WFC server, like a temporary resident, or like someone renting a function room for a meeting, but when the host leaves, or is removed from the venue, the whole party has to leave too.
And Sleeve - sorry to hear that, it happened to me a few days before I did get past wave 15... I wasn't happy either.
I've also been spending a fair bit of time on Escalation, trying to see if I can get to wave 15 as a solo player - have managed to get to wave 11 so far, twice, in Broken Hope. (the first time, I died because I couldn't hold off taking a leak any longer...)