It would be easier to add some CSS, It think there may have been additional follow-up commits to those.
This would solve it. It uses an inline SVG of the correct logo to replace the existing one and updates the background color.
.btn-social.facebook {
background: #4267b2;
svg {
display: none;
}
&:before {
width: 15px;
height: 15px;
content: url('data:image/svg+xml; utf8, <svg aria-hidden="true" data-prefix="fab" data-icon="facebook" class="svg-inline--fa fa-facebook fa-w-14" xmlns="http://www.w3.org/2000/svg" width="15" height="15" viewBox="0 0 448 512"><path fill="white" d="M448 56.7v398.5c0 13.7-11.1 24.7-24.7 24.7H309.1V306.5h58.2l8.7-67.6h-67v-43.2c0-19.6 5.4-32.9 33.5-32.9h35.8v-60.5c-6.2-.8-27.4-2.7-52.2-2.7-51.6 0-87 31.5-87 89.4v49.9h-58.4v67.6h58.4V480H24.7C11.1 480 0 468.9 0 455.3V56.7C0 43.1 11.1 32 24.7 32h398.5c13.7 0 24.8 11.1 24.8 24.7z"/></svg>');
}
}