A CodePen by Stephen Burgess. Sexy Social Buttons - CSS only animation effect on hover of these simple but fun social buttons.

index.html

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

script.js

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

style.css

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;
}