Font Awesome Icons not visible to some users

I had two persons telling me they can't see the social media in the top right of my website due to font-awesome. Do you know if there's a fix for this?

<nav class="navbar navbar-default navbar-fixed-top m-d-2">
    <div class="container-fluid">
        <!-- Brand and toggle get grouped for better mobile display -->
        <div class="navbar-header">
            <button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#nav-main" aria-expanded="false">
                <span class="sr-only">Toggle navigation</span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
            </button>
            <a class="navbar-brand" href="index.html">Mona Jalal</a>
        </div>
        <div id="top-menu" class="collapse navbar-collapse" id="nav-main">
            <ul class="nav navbar-nav">
                <li class="active"><a class="nav-item nav-link page-item" href="education.html">Education</a></li>
                <li><a class="nav-item nav-link page-item" href="coursework.html">Coursework</a></li>
                <li><a class="nav-item nav-link page-item" href="teaching.html">Teaching</a></li>
                <li><a class="nav-item nav-link page-item" href="projects.html">Projects</a></li>
                <li><a class="nav-item nav-link page-item" href="honors.html">Honors & Awards</a></li>
                <li><a class="nav-item nav-link page-item" href="experience.html">Experience</a></li>
            </ul>
            </ul>
            <ul class="nav navbar-nav navbar-right">
                <li><a class="navbar-brand pull-sm-right" href="https://github.com/monajalal"><i class="fa fa-github fa-lg"></i></a></li>
                <li><a class="navbar-brand pull-sm-right" href="https://twitter.com/MonaJalal_"><i class="fa fa-twitter fa-lg"></i></a></li>
                <li><a class="navbar-brand pull-sm-right" href="https://www.linkedin.com/in/mona-jalal"><i class="fa fa-linkedin fa-lg"></i></a></li>
                <li><a class="navbar-brand pull-sm-right" href="http://stackoverflow.com/users/2414957/mona-jalal"><i class="fa fa-stack-overflow fa-lg"></i></a></li>
                <li><a class="navbar-brand pull-sm-right" href="mailto:jalal@cs.wisc.edu"><i class="fa fa-envelope fa-lg" aria-hidden="true"></i></a></li>
                <li div="fa-item"><a class="navbar-brand pull-sm-right" href="https://www.instagram.com/mona_of_green_gables"><i class="fa fa-instagram fa-lg"></i></a></li>
            </ul>
        </div>
    </div>
</nav>