So it appears that no one is unassailable.
“Looking at the overall search engine market from May 2010 to May 2011, Compete found that Google has lost close to 16 percent of its share, dropping to 63.6 percent from 73.9 percent. At the same time, Microsoft grew its share by 75 percent, jumping to 17 percent from 9.7 percent.”
Bing on the right and Facebook on the left. Stuck in the middle with Google.
So don’t assume your users are using Google exclusively.