I'm glad I got to use these credits to see how inefficient this form of advertising is.
First, while Bing customer service is good, the delivery of the ads seemed inconsistent and doesn't update for three hours after the fact. There were also huge unexplained gaps in impressions and the ad took forever to be approved. The CTR was actually decent for dollars spent.
Facebook literally shut down one of my accounts, said my account had been hacked (it wasn't), closed my payment option, took FOREVER to approve ads. FINALLY, it started using the coupon. The result was about $31 spent for 170,000 impressions and a total of 74 clicks (lol).