Social Media Icons Do Not Display

Expected behavior The navbar uses:

      <ul class="navbar-nav nav-flex-icons">
        <li class="nav-item">
          <a class="nav-link"><i class="fab fa-facebook-f"></i></a>
        </li>
        <li class="nav-item">
          <a class="nav-link"><i class="fab fa-twitter"></i></a>
        </li>
        <li class="nav-item">
          <a class="nav-link"><i class="fab fa-instagram"></i></a>
        </li>
      </ul>

So, I should see social media icons to the right.

Actual behavior The cursor changes to pointer on hover, but no icons are displayed

Resources (screenshots, code snippets etc.)

I cannot find fa-facebook-f, fa-twitter, or fa-instagram in the mdb.css document. Should these classes be present in the document?

Navbar Code:

<!--Navbar -->
<header>

  <nav class="navbar fixed-top navbar-expand-lg navbar-dark scrolling-navbar">
    <a class="navbar-brand" href="#"><img src="img/puzzle.png" class="puzzle-logo" alt="puzzle-logo"></a>
    <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent"
      aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
      <span class="navbar-toggler-icon"></span>
    </button>
    <div class="collapse navbar-collapse" id="navbarSupportedContent">
      <ul class="navbar-nav mr-auto">
        <li class="nav-item active">
          <a class="nav-link" href="#">Home <span class="sr-only">(current)</span></a>
        </li>
        <li class="nav-item">
          <a class="nav-link" href="html/about.html">About</a>
        </li>
        <li class="nav-item">
          <a class="nav-link" href="html/contact.html">Contact</a>
        </li>
      </ul>
      <ul class="navbar-nav nav-flex-icons">
        <li class="nav-item">
          <a class="nav-link"><i class="fab fa-facebook-f"></i></a>
        </li>
        <li class="nav-item">
          <a class="nav-link"><i class="fab fa-twitter"></i></a>
        </li>
        <li class="nav-item">
          <a class="nav-link"><i class="fab fa-instagram"></i></a>
        </li>
      </ul>
    </div>
  </nav>

</header>
<!--/ Navbar -->