Oneof  the things you want to know when starting the seo campaign is what search engines you would like to target. Here are few ­reasons you might want to know that :

1. You want to roughly estimate the traffic that's going to be coming to your website
2. You want to estimate the cost associated with the campaign
3. You want to know the link building strategy that you will be using

Market share split in Canada and U.S.

U.S. traffic share split according to Hitwise estimates:
Google: 67.9%
Yahoo: 20.3% (unrounded, 20.28%)
Microsoft: 6.3% (unrounded, 6.26%)
Ask: 4.2% (unrounded, 4.17%)
Other search engines: 1.3%

In Canada the share is split differently:
Google holds about 70-80% of all searches
Yahoo holds about %15
MSN (also, live.ca and bing.ca/.com) holds about %5
Other search engines: 2-3%

Associated costs

Obviously targeting Google in any case is going to be more expensive than targeting yahoo or msn. However, there may be exceptions for different industries. You should also keep in mind than men prefer to use Google and women prefer Yahoo, especially, keeping in mind Yahoo's last advertising campaigns on TV targeting women segment of the market.

Link Building Strategies

When planning to take leading positions you should keep in mind that different search engine rely on different factors when assigning positions to each website. It is well known that Google loves authority and relevancy. So, for example, if the website has a lot of backlinks, good PR, and ranks for similar keywords that you are targeting – that is differently the website that you want to be linking to you.

On the other hand MSN search algorithm does not care about these things. What MSN looks at is a sheer volume of links and the keyword presence in the titles, meta tags, and url. That is why ranking on msn is always faster and easier, but unfortunately achieved rankings never generate traffic that is even comparable to Google's.

Dealing with Yahoo is like dealing with random hybrid of both search engines. Yahoo cares about the authority of the actual website, but also gives a lot of weight to the titles and meta tags.