Forum Posting is another effective way of getting fast backlinks thru social interaction. A majority of DoFollow forums allow signatures where you can put links back to your site.
An advantage of forum posting is that you can register for related forums and learn more about the type of business that you are in. For example, I have an SEO site, so I registered at SEO forums not only to build backlinks but to increase my SEO knowledge as well.
However, spam should be avoided in forum interaction. Forum moderators sometimes ban users and IP addresses who they believe are just posting spam content. Be sure to read the rules and regulations of the forums that you are joining to avoid getting banned.