icon in menu

ok, one more question.
I want to replace a tab in the menu with the icon. I’ve done this before in other sites with css like this

.menu-facebook {
text-indent: -9999px;
background-image: url(url to the picture) !important;
background-repeat: no-repeat !important;
margin-left: 4px !important;
width: 40px;}

but it can’t seem to be working at all here. Any ideas?

thank you!