customer-chat-facebook-defer.js

<script defer>
    document.addEventListener('DOMContentLoaded', function() {
        setTimeout( function () {
            window.fbAsyncInit = function() {
                FB.init({
                    xfbml            : true,
                    version          : 'v7.0'
                });
            };
            (function(d, s, id) {
                var js, fjs = d.getElementsByTagName(s)[0];
                if (d.getElementById(id)) return;
                js = d.createElement(s); js.id = id;
                js.src = 'https://connect.facebook.net/en_US/sdk/xfbml.customerchat.js';
                fjs.parentNode.insertBefore(js, fjs);
            }(document, 'script', 'facebook-jssdk'));
        }, 1500);
    }); 
</script>

ref: https://stackoverflow.com/questions/15349867/defer-loading-of-facebook-like-button-script