HI farbodfan,

You can add social network or other icons in this form:

<i class="fa fa-{icon-name}"></i> where {icon-name} is the icon from font awesome collection which can be found here:

https://fontawesome.com/v4.7.0/icons/

Here is how to find the icon name:

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Please go to Appearance > Menus and don't forget to tick Custom Links option:

(view on new page)

Then add it to menu item in Appearance > Menus, you can use Custom Link or current pages on your site.

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Have a great Sunday!