Using Open Source Software means more than using the code, especialy if you are a high profile user like Google. It’s a relationship. In addition to free code, a Big Co using OSS may gain access to positive publicity, evangelists, early adopters.
However this comes at a cost. You have to favor this relationship. Google has not done bad in this field. Most of their products will work flawlessly in OSS browsers and they advertized their use of OSS when OSS needed this publicity. Do they need to give back more? I think they could.
Read Google Under Fire for Open Source Greed at Threadwatch.org.