Same here, but I haven't seen much MSN content traffic this week, either. I also had problems with search for the past few weeks not even getting impressions, and I should've had thousands of daily impressions according to estimates. After talking with support for two weeks I'm redoing the campaigns and trying again.
If it's a serious drop in traffic and you have no idea what the problem could be, I'd highly recommend calling Adcenter support. Whenever I called they were always very eager to help, even if they didn't quite solve my problem.