.facebook-btn {
  display: inline-block;
  width: 150px;
  height: 35px;
  background-image: url('/Plugins/ExternalAuth.Facebook/Content/Images/facebook_logo.png') !important;
  background-repeat: no-repeat;
  background-size: 100% 100%;        /* 图片强行填充整个按钮 */
  background-position: center;       /* 居中显示 */
  background-color: transparent;     /* 去掉背景色 */
  border: none;
  text-indent: -9999px;              /* 隐藏文字 */
  cursor: pointer;
}