Facebook has quietly announced on their website that with the release of Graph API v2.3, the Like Box plugin for pages is deprecated and will stop working on June 23rd 2015. They advise that you should use their new Page Plugin instead.
The Like Box is a variant of the embeddable Like Button that allows you to Like a page from other sites. The embed would also show off some of the other people who liked a given Page.
The company recommends you instead use the newer Page Plugin, which functions similarly, but looks a little cleaner and is able to pull in content feeds from your Facebook Page if you want.
Keep in mind that you’ll have to add in the newest Graph API and remove the old code to enable the newer plugin.