This article
http://www.insidefacebook.com/2010/10/15/as-source-for-current-facebook-employees-google-has-big-lead-on-yahoo-microsoft-oracle/ suggests there are 277 ex-Googlers at Facebook. (There are reduced numbers from other big tech employeres).
--stock options: Facebook is/was pre-IPO. If you want to get rich as an engineer you would work there. You will never get that rich at Google.
--freedom: Google is a large company and it is hard to get stuff done. Facebook is small.
--Google is perceived as no longer being the place where the best work.
+1 insightful
There is another possibility: automatic search (attempting to find relevant pages for everyone) has reached a plateau in terms of performance and if you want to do better, you will need to employ raters. Clearly Google would like to find "the next best thing" in Search, but that sounds quite uncertain. Employing lots of people is a much surer way to improve results.
Get hold of portable property. -- Charles Dickens, "Great Expectations"