Using the fontawesome-all.css file: Changing the "Brands" font-family from "Font Awesome 5 Free" to "Font Awesome 5 Brands" fixed the issues I was having.
I can't take all of the credit - I fixed my own local issue right before looking at the CDN version: https://use.fontawesome.com/releases/v5.0.6/css/all.css
They've got the issue sorted out on the CDN as well.
@font-face {_x000D_
font-family: 'Font Awesome 5 Brands';_x000D_
font-style: normal;_x000D_
font-weight: normal;_x000D_
src: url("../webfonts/fa-brands-400.eot");_x000D_
src: url("../webfonts/fa-brands-400.eot?#iefix") format("embedded-opentype"), url("../webfonts/fa-brands-400.woff2") format("woff2"), url("../webfonts/fa-brands-400.woff") format("woff"), url("../webfonts/fa-brands-400.ttf") format("truetype"), url("../webfonts/fa-brands-400.svg#fontawesome") format("svg"); }_x000D_
_x000D_
.fab {_x000D_
font-family: 'Font Awesome 5 Brands'; }_x000D_
@font-face {_x000D_
font-family: 'Font Awesome 5 Brands';_x000D_
font-style: normal;_x000D_
font-weight: 400;_x000D_
src: url("../webfonts/fa-regular-400.eot");_x000D_
src: url("../webfonts/fa-regular-400.eot?#iefix") format("embedded-opentype"), url("../webfonts/fa-regular-400.woff2") format("woff2"), url("../webfonts/fa-regular-400.woff") format("woff"), url("../webfonts/fa-regular-400.ttf") format("truetype"), url("../webfonts/fa-regular-400.svg#fontawesome") format("svg"); }
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