Facebook Like Button Takes Over Share Button Functionality:
After months of updates to its Like button, Facebook has released an update that fundamentally changes the button’s functionality to that of a Share button. Now after hitting the Like button, a full story with a headline, blurb and thumbnail will be posted to your profile wall. You’ll also be given an option to comment on the story link. Previously, only a link to the story would appear in the recent activity, often going unnoticed by users.
Basically it means that when a visitor to your website clicks a Facebook "like" button on your site, it sends a link to your webpage to their Facebook feed.
In the past, that only happened when people clicked a "share" button.
Having the "Like" button on every page of your site is now another opportunity for great word of mouth marketing.
Here's an example from my Facebook wall, where I just clicked the "Like" button on a webpage: