Backlinking is two-fold. 1) Search engines rank better sites with many backlinks. 2) Users can find your link around the web and follow it to your site. If you happen to have a website based on "difficult" keywords, i.e. few chanches of ranking, probably you would like to concentrate on building backlinks that carry the most "direct users" value. Which kind of links would you build then? Youtube? Twitter? Facebook? Delicious? what else? Thank you.