Looking for icon-facebook similar to homepage of gohugo.io

In the footer of http://gohugo.io/ there are twitter and github icons.

They are using below classes:

class="icon-twitter icon-2x"
class=“icon-octocat icon-2x”

I want to add facebook icon in similar fashion.

I added class=“icon-facebook icon-2x” to the “index.html” but it’s not working.

It’s an icon font, which you have to have loaded via CSS first. For an easy implementation of your own, check out Font Awesome.