Social buttons (rendered in the footer) are great for having a small hub with all your social footprint. In order to change them, jump to _config.yml and edit the social list:
social:
- title: "github"
url: "https://github.com/"
- title: "linkedin"
url: "http://linkedin.com/"
- title: "youtube"
url: "http://youtube.com/"
- title: "rss"
url: "/feed.xml"
The titles of the social link are used to find the icon in Font Awesome.
To look for an icon, search for one at the Font Awesome icon gallery and put the icon name (without fa-) as title.