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.
Their page plugin in similar to the old like box but is a little more visual and allows you to embed a simple feed of content from a Page into your websites. If you want to add the new page plugin you can visit https://developers.facebook.com/docs/plugins/page-plugin but you will required to remove the old code and add in the new SDK. The settings on the new page plugin is fairly similar to the old like box as you can see below.