div#social-links {
    margin: 0 auto;
    max-width: 500px;
}
div#social-links ul li {
    display: inline-block;
}          
div#social-links ul li a {
    padding: 20px;
    border: 1px solid #ccc;
    margin: 1px;
    font-size: 30px;
    color: #222;
    background-color: #ccc;
}