The version of Font Awesome currently included with Moxie is v3.2.1. The font class structure changed in v4 of Font Awesome, but you need to use the old v3 class structure.
Font Awesome v3.2.1 icons → http://fortawesome.github.io/Font-Awesome/3.2.1/icons/
Font Awesome v3.2.1 example usage → http://fortawesome.github.io/Font-Awesome/3.2.1/examples/
The class structure changes in v4 of Font Awesome require modifications to styles across the entire Moxie theme. We haven’t completed that work yet, but once we have we will update the version of Font Awesome that is integrated into Moxie.