LinkIn/Plaxo compared to Facebook/MySpace

Thinking about the types of social media sites we've utilized, I find LinkIn and Plaxo to appear more professional than Facebook/MySpace.  Examples on the sites are more formal, not as casual as Facebook.  Topics seemed more directed, not the casual conversation seen on Facebook.  It is understandable that professionals might use these sites, however, I really don't understand if you are suppose to direct a potential employer to your site, or if employers looking to hire, find people through the site.  If the latter is the case, which I am led to believe, then I really don't understand how they do the search.  I was surprised at the number of people I know who were members of these sites, but I wonder how much they utilize the site or keep it updated.  Just some thoughts . . .