werty - 5:59 pm on Feb 28, 2013 (gmt 0)
Google - 80%-90%
Bing/Yahoo - 5-10%
Facebook - 5-10%
Really depends for me though. Some stuff does not work well on Facebook, while other stuff does better there than elsewhere.
I always start with a Google campaign though for research and setting the standard/structure. Then I import it into Bing. Facebook varies by client.