Currently: No Facebook ad coupons in 2012 to my knowledge

A couple of pages with coupons: ALL ARE UNVERIFIED, please let me know if they don’t work for you.

http://www.netpaths.net/blog/50-facebook-coupon-advertising-credit/

http://www.adscoupons.com/$50-Facebook-Ad-Coupon

http://www.adcouponcode.com/

This page will be updated as they become verified or known to not work.

Facebook ads coupons are usually given as part of promotions and to encourage new advertisers. To find more, look at the official guide from Facebook about ad coupons: http://www.facebook.com/help?page=872

That page is also useful, especially for problems with using them and finding out about new ones.