Google is biggest one of all and it can send multiple times more traffic then any other search engine. So, if I have concentrate at any other search engine which one it should be?
Its not wise to concentrate at any particular search engine. I have seen this many times people may invest fortune but the outcome still remains the same. Instead focusing on them focus on your content and getting quality links, the search engines will come around eventually.
Build quality content and links & forget about any search engines. They will send traffic anyway if your site is good.
Focus on Bing its the second largest.
Just focus on content. Focusing on any specific search engine is stupid.