Unicode via CSS :before

In CSS, FontAwesome unicode works only when the correct font family is declared (version 4 or less):

font-family: "FontAwesome";
content: "\f066";

Update - Version 5 has different names:

Free

font-family: "Font Awesome 5 Free"

Pro

font-family: "Font Awesome 5 Pro"

Brands

font-family: "Font Awesome 5 Brands"

See this related answer: https://stackoverflow.com/a/48004111/2575724

As per comment (BuddyZ) some more info here https://fontawesome.com/how-to-use/on-the-desktop/setup/getting-started

Fileformat.info is a pretty good reference for this stuff. In your case, it's already in hex, so the hex value is f066. So you'd do:

content: "\f066";

The escaped hex reference of  is \f066.

content: "\f066";