How About Ads of Facebook and StumbleUpon? I used Google Adwords for about 3 years and never used social networds ads. StumbleUpon ad seems cheap, $0.05/Visiter.
I wouldnt really use facebook ads unless your prepared to spend a few $/£ a day, unless your willing to pay around £/$0.30 an add, it wont show up much, i spent £8 in the space of about 5 minutes the other day, and didnt get much from it (I was using there PPC ads)...Google adwords are very good though, managed to get a fair amount of traffic from using them
If what you are targeting is a focused niche you can get away with being profitable on Facebook running on CPC. If you want to make it big you'll have to go on to CPM and CPM is tricky business. You can lose a lot and you have to be on the ball.
NEVER us facebook adds to promote a product right away, use it to build a relationship with potential buyers. most likely you are going to waste ur money. People in stumble uppon is a place for people that like cool stuff... therefore you you want to promote intresting stuff.
If used correctly Facebook can bring many digits of profits a day promoting CPA offers. You can get very cheap and targeted clicks using Facebook.