While both can be pay per click campaigns, it's apples and oranges for many reasons.
First you have to consider the audience. With AdWords you are targeting searches which is an action... I am looking for X and thus you are a potential solution. As a result AdWords offers a great chance to convert to a purchase but has a limited population and fierce option competition from both other vendors and the user themselves.
With Facebook you are reaching demographics and have no idea if they are actively considering an action or the category you are in at all. Thus you have a tremendous opportunity to impact discovery and reach people who may not be comparing let alone thinking about what you offer.
The mechanics of the site are also fundamentally different. On Google you are looking to leave -- they are a portal to another destination and a one hit visit that, if you don't convert on your own site, is gone. On Facebook you are looking to engage and communicate with friends; links and outbound clicks are generally limited to seeing what others share. So if you try to drive a ton of website traffic as your primary goal on Facebook, you leave a lot of opportunity on the table vs driving up likes or offering on-site programs that build more around affinity and a long term conversation.
Both can have killer results but they have to be optimized to what they are and what the user expects in the time and place. If you take a "it worked on A thus it should work on B" attitude, you're not giving things a chance to succeed.
[p.s. Moving this to the Internet Marketing forum]