Here's is a paragraph describing how Google legally dodges tax:
"The Dublin subsidiary sells advertising globally and was credited by Google with 88 percent of its $12.5 billion in non-U.S. sales in 2009."
http://www.bloomberg.com/news/2010-10-21/google-2-4-rate-sho...
So it seems a bit silly to focus solely on US searches. These are global markets we are talking about.
EDIT: link
Bing results are total garbage outside the US at the moment, so it's no surprise that their share would be low. And likewise it makes sense for MS to concentrate on reaching a critical mass in a single market first, and only worry about the rest of the world later.
Their total 2009 revenue was much higher, $23.6B (URL: http://investor.google.com/earnings/2009/Q4_google_earnings....) making non-US revenue a pretty big chunk at 53%.
Please stop using the phrase "tax burden". It's loaded language and not-so-subtly encourages a biased interpretation. It would be like saying "tax opportunity"; i.e., "an opportunity to pay tax".
I don't think that's accurate. I read that article as saying that 88% of the $12.5 billion it made outside the US was booked in Dublin. Not that 88% of their revenue is from outside the US.