The Send button drives traffic by letting users send a link and a short message to the people that would be most interested. They don’t need to leave the web page they’re on or fill out a long, annoying form. Compared to the alternatives, the Send button has fewer required steps, and it removes the need to look up email addresses by auto-suggesting friends and Groups. (Facebook Developer Blog)
Facebook is also planning on integrating their real time metrics system within the Send button, which will make the feature much more attractive than traditional email sharing options.
Currently users have to manually install the Send button upon their sites until a geek free solution can be devised, although if anyone knows of an easier way of installing the code (without placing it within the footer), feel free to comment below.