Facebook's ad revenue share has been increasing, while Google's has been decreasing. In just two years, Google has gone from owning 10x as much ad revenue share as Facebook to just 2x [0]. Facebook has much more data on its users (age, location, school, etc) than Google which allows advertisers to target their ads much easier. It seems like Facebook is better if you want to narrowly reach a specific demographic since they know so much about their users, whereas Google is better if you just want to broadly reach a lot of people since they have such a dominant presence on the web. It depends on what you want as an advertiser, but there's definitely people who would prefer to spend their advertising dollars on a specific demographic.
I'm baffled to think that one company providing its own advertising for its own website is constituting such a huge portion of web ads in general, considering there are thousands of websites using Google Adsense that somehow live off the ad revenue.
[0]: http://www.emarketer.com/Article/Driven-by-Facebook-Google-M...