In font awesome 5 the font family declaration has changed to the following.
Make sure font-weight is also set to 900.
/* For method Web Fonts with CSS */
font-family: "Font Awesome 5 Free";
font-family: "Font Awesome 5 Pro";
font-family: "Font Awesome 5 Brands"; /* this one works best */
/* For method SVG with Javascript*/
font-family: "Font Awesome 5 Solid";
font-family: "Font Awesome 5 Regular";
font-family: "Font Awesome 5 Brands";
font-family: "Font Awesome 5 Light";