Facebook like button is a component of its social plugins. It is a great tool of driving traffic to your site. Mashable has a report of increasing referral traffics by 50% by using this Like button.
Considering it as a good tool to drive traffic, I have developed a tiny extension for PunBB bulletin board. When you install this extension, you will find a Like button at the top of Topic View (viewtopic.php). It will also show number of persons liked the topic.
When someone likes a topic, it is showed on their profile page hence to his/her friends. This will open a great window of huge traffics to your forum.
Photo Courtesy: Mashable.com