Advertising through your competitors may seem like a backwards way to market your product or service, but Yahoo isn't the only company doing so. Many of Google's competitors advertise through Google. Bing, a Microsoft owned search engine advertises through Google/Google's sources. I think that this kind of advertisement is a very smart way for companies to advertise. Yahoo for example, isn't a direct competitor of Facebook. Yahoo offers services that Facebook doesn't, and likewise, Facebook offers services that Yahoo doesn't.
Right now, Facebook is the single largest social-networking service online. Although Yahoo was around first, Facebook has managed to tap into a whole new network of consumers that may not have been online before. If Yahoo can get just a few of those new consumers to join their service too - than the advertisement is successful and it will be. My Nana never had a computer until Facebook came out. She wanted a way to see what her children and grandchildren were up to, and Facebook was a very easy way for her to do that. Now that she has a computer she has been learning how to surf the web and use things like e-mail and follow the stock exchange online. Her e-mail is through Yahoo's free e-mail service. I have no way to prove that Facebook is how she found out about Yahoo, but I can be sure that Facebook is the reason she was interested in getting an e-mail at all.
Does anyone else know of companies who have succeeded in advertisement through competitors?