To add Yelp icon to the social media icons in DIVI, add the following code to includes/social_icons.php before </ul>:

<li class="et-social-icon et-social-yelp">
<a href="yelp link goes here‎" class="icon">
<span><?php esc_html_e( 'Yelp', 'Divi' ); ?></span>
</a>
</li>

then add the following code to custom css box in Divi>theme options:

.et-social-yelp a.icon:before { content: url("Yelp icon url here"); }