one of their top advertisers is gonna have to pull out too.
Consider changing to a PPC or CPM model with them instead of flat fee or whatever you're using now. If it's well targeted traffic you would still benefit from a presence there as long as the costs were proportionate to performance.
in the real world you need to do some marketing to help things along.
"Build it and they will come" only starts to work after you've pushed long and hard.