Targeting social networking sites with your content and maybe affiliate linked articles can be much harder to get the traffic you need to make money. People using all the social networking services are likely to be bloggers themselves. In fact most people that read blogs do blogging themselves. And these are the people that know the game of blogging. They wont click those ads and most bloggers themselves know how to take the affiliate out of the marketing by removing your referral link and just going to the site itself. Then you make no money online and have to stick with your day job.
On the other hand targeting search engines means that the people coming to your site is already looking for what you have on your site. And most of the time people have advertisements and affiliates matching your content. That means more people will click on your affiliates because that is after all why they came to your site
