I've just finished trying Facebook... what a crappy network. Horrible conversions, horrible CTR, long waits for ad approval, etc. I'm also trying Bidvertiser which has sucked for me. Long waits to approve ads (do they even approve on weekends?), a bit expensive for the click quality, etc. Adwords is of course the best, but it's extremely expensive for good keywords. I find their slow stats system aggravating as I like to monitor my results closely. I've been thinking about trying Adbrite, but no coupons and $5 to start has me skeptic.