fontawesome.com facebook icon code example

Example 1: fa fa-facebook

<i class="fa fa-facebook-square" aria-hidden="true"></i>

Example 2: font awesome facebook

Font Awesome 5:
 
Letter F:  <i class="fab fa-facebook-f"></i>
Round:     <i class="fab fa-facebook"></i>
Square:    <i class="fab fa-facebook-square"></i> 
Messenger: <i class="fab fa-facebook-messenger"></i>