Just seeing a momentous drop off in both impressions & searches, started late yesterday - anyone else?
Calc Richmond, the reason for this could be as simple as having made changes to your ad copy or URL. Did you edit your ad(s) just before the drop?
Substantially reduced traffic can occur after an ad is edited, as it will then only show on Google until it has been reviewed and approved again.
Once approved, it may once again be sent to the search and content networks. The difference between Google alone and Google plus potentially hundreds of other sites can amount to a considerable amount of traffic.
AWA