It is because you have removed the previous link and added a new one. Try to add this CSS class fb-menu-link to your menu item and replace your previous code with this one

.fb-menu-link > a {
	color:transparent!important;
  width:30px;
  background-size: 20px;
  background: url(http://snowcon.no/wp-content/uploads/2016/10/FB-f-Logo__blue_58.png) no-repeat center center;
}

.fb-menu-link > a > i {
	display:none;
}