Google is by far the most used search engine.It provides search results not only on it's home page, but powers other search engines' results as well.
the very reason google is on top is because it finds its origin as a search engine. the others were into all sorts of other things.. like news, reports, emails etc. gmail came in much later and was successful because of the high amout of disk apace it provided.
Google is by far the most heavily used search engine and it is miles ahead of the second position which is right now witnessing a good battle between Yahoo and Bing! Regards, RightMan