On couple of my websites I recently installed Addthis or ShareIT button which gives the option to the visitor to share the website, page with friends, social networks etc, and the results have been simply amazing.
Especially because this is automarketing and now a days social networks like facebook, twitter etc have lot many visitors. I have seen that many of my websites no get visitors from such networks and the analytics of my Addthis account shows that a no. of visitors are using the service to share it with friends.