.btn-social
border: 1px solid red
background-repeat: no-repeat
background-position: center
.btn-social--fb
background-image: url(...)
.btn-social--twitter
background-image: url(...)
.btn-social
border: 1px solid red
.btn-social--fb
background: url(...) no-repeat center
.btn-social--twitter
background: url(...) no-repeat center