Many people are already familiar with MySpace, Facebook, LinkedIn and similar social media websites.  Because the goal of these sources is to help people develop online relationships based on common interests, they can be a very powerful means of exposing a site to a large target audience.  Creating profiles through these sources - as well as participating in discussions, groups, message boards and blogs on other sites related to your industry - is a powerful website marketing tool. 

For example, treatment center for eating disorders could create an online discussion at Bebo that is dedicated to providing information about eating disorders.  That discussion might include everything from how to recognize that someone may have an eating disorder to the physical repercussions of eating disorders if left untreated.  Including a link to the treatment center's site with each blog posting or message board response is great exposure for the treatment center's website.  Not only does it advertise the site to whomever may be reading the information, but it can help to increase the link popularity of the site with the search engines.  

The same would be true of the same business owner participating in existing discussions about eating disorders on other eating disorder related blogs, message boards, etc.  Posting useful information and discussing the benefits of seeking treatment would be a great way build a rapport with the people who may be in desperate need of treatment.  If a live link to the treatment center's website is included in that information, that's even better - the treatment center would have then used someone else's blog, message board or forum to increase the link popularity of their site, as well as giving whomever is reading the information a direct link to them.  Again, a win-win.