Go to your site collection > site settings page and activate the following Site Features (in this order)
- Search Server Web Parts and Templates
- SharePoint Server Publishing Infrastructure
Go to a blog post and edit the page.
Add the Content Editor Web Part and edit it's source toi iFrame your content.
Paste your iFramed Facebook Source code
<iframe src="//www.facebook.com/plugins/like.php?href=https%3A%2F%2Fdevelopers.facebook.com%2Fdocs%2Fplugins%2F&width&layout=standard&action=like&show_faces=true&share=true&height=80" scrolling="no" frameborder="0" style="border:none; overflow:hidden; height:80px;" allowTransparency="true"></iframe>
Change Chrome Type to None (by editing the web part) and the Title "Content Editor" won't show.
Stop editing, and you're done!
Editing a Blog Post by adding a Web Part actually updates the Blog Post Template to include the web part on all posts. See the below images on two different posts, where I've only updated one post (template).