Facebook introduced the Like button to websites outside Facebook in 2010.
The company unveiled the social plug-in at its F8 developer conference in April 2010. Website owners could add the button so visitors could express approval and connect a page or article with Facebook activity.
The button rapidly became one of the web’s most recognizable interface elements. It allowed Facebook to extend its identity and social graph beyond facebook.com, while publishers gained a simple way to encourage sharing and engagement.
The external Like button should not be confused with the original Facebook-only Like feature, which launched earlier. The question concerns the button’s expansion to third-party websites, not the first appearance of liking content inside Facebook.