FB and Adwords is like "chalk and cheese"... in that they are both very different with requiring different approaches...
With adwords you are targeting people who are looking to buy, provided that you use the correct keywords... For example lets take the dog training niche... If I use the keyword "dog training" - thats a bit too high level given the intent behind it could range from a user looking for free resources so chances are you'll lose a lot of clicks for nothing.... If however I used the keyword "dog training DVD" - you can almost see that person sitting their with their wallet eager to buy... So your keywords are essential as this is the difference between ROI and fiscal losses...
With FB - nobody on social networks are looking to buy... They are their to socialize/connect with friends etc... So sales ads appearing on their walls is generally speaking very annoying... You can even see for yourself with comments below such ads... I've tried direct selling on FB and for me it was a total waste of time money and effort... What works on FB is to use that for list building or through building up engagement/social shares etc through interesting/engaging content.... with sales being a longer term strategy primarily through email marketing...
In sum:
Adwords Advantages: Connect to eager buyers. Great for direct sales.
Adwords Disadvantages: Many policies e.g. "info harvesting policy", "bridge policy" - in otherwords dont bother with list building, affiliate marketing due to these policies. User safety is another one whereby no exaggerated claims, essential need for disclaimers and disclosures etc...
FB Advantages: Great for list building, great for targeting demographics e.g. gender, location, interests etc
FB Disadvantages: Terrible for direct sales.
For me both are great platforms... The important thing is to learn how to use each one for your maximum benefit...