like the post above, the release of call of the dead made a sheite load more people play zomibies.
Before call of the dead more then less then 100k people would be on zombies and no call of the dead seems to of aged on everyone and got very boring very quick. There was probably just a lot of people playing it to try and do the easter egg.
I don't think it's because Call of the Dead got Boring. I have played it over 100 times and I still love it. After playing ascension once I got bored of it because it is so easy compared to call of the dead. maybe people are just waiting for shangri la.
I saw there were half a million people playing CoTD as well as Ascension yesterday, and like the person above, I don't like playing with randoms so much because of the host leaving + host migration. All of those problems are solved by doing private match (of course you could find some good randoms and party them up and get into a private match which is what I do).