<a href="https://twitter.com/minimalmonkey" class="icon-button twitter"><i class="icon-twitter"></i><span></span></a>
<a href="https://facebook.com" class="icon-button facebook"><i class="icon-facebook"></i><span></span></a>
<a href="https://plus.google.com" class="icon-button google-plus"><i class="icon-google-plus"></i><span></span></a>

html {
  font-size: 20px;
}
body {
  background-color: #ccc;
  padding: 50px;
  text-align: center;
}

/* Wrapper */
.icon-button {
  background-color: white;
  border-radius: 3.6rem;
  cursor: pointer;
  display: inline-block;
  font-size: 2.0rem;
  height: 3.6rem;
  line-height: 3.6rem;
  margin: 0 5px;
  position: relative;
  text-align: center;
  -webkit-user-select: none;
     -moz-user-select: none;
      -ms-user-select: none;
          user-select: none;
  width: 3.6rem;
}

/* Circle */
.icon-button span {
  border-radius: 0;
  display: block;
  height: 0;
  left: 50%;
  margin: 0;
  position: absolute;
  top: 50%;
  -webkit-transition: all 0.3s;
     -moz-transition: all 0.3s;
       -o-transition: all 0.3s;
          transition: all 0.3s;
  width: 0;
}
.icon-button:hover span {
  width: 3.6rem;
  height: 3.6rem;
  border-radius: 3.6rem;
  margin: -1.8rem;
}
.twitter span {
  background-color: #4099ff;
}
.facebook span {
  background-color: #3B5998;
}
.google-plus span {
  background-color: #db5a3c;
}

/* Icons */
.icon-button i {
  background: none;
  color: white;
  height: 3.6rem;
  left: 0;
  line-height: 3.6rem;
  position: absolute;
  top: 0;
  -webkit-transition: all 0.3s;
     -moz-transition: all 0.3s;
       -o-transition: all 0.3s;
          transition: all 0.3s;
  width: 3.6rem;
  z-index: 10;
}
.icon-button .icon-twitter {
  color: #4099ff;
}
.icon-button .icon-facebook {
  color: #3B5998;
}
.icon-button .icon-google-plus {
  color: #db5a3c;
}
.icon-button:hover .icon-twitter,
.icon-button:hover .icon-facebook,
.icon-button:hover .icon-google-plus {
  color: white;
}

// Uses rem for size so go ahead and change the font-size of html and see the buttons get bigger (or smaller...) :)