You have you CSS a little messed up. You have the background on the container, not the anchors.
.container
{
width:1247px;
height:30px;
background-color:#444444;
}
.container a
{
background: transparent url('http://i.imgur.com/s5rf9GY.png') no-repeat;
height: 30px;
width:30px;
display:block;
float:left;
text-indent:-9999px
}
.container .Twitter
{
background-position: -491px 0;
}
.container .Plus
{
background-position: -527px 0;
}
Check the fiddle http://jsfiddle.net/Tn4F4/3/