This small gist allows creating "Share buttons" (or something else) which fade in and while the content is in the viewport.

Fade-in share buttons.css

.post-sharing-side {
  position: fixed;
  top: 50%;
  right: 50%;
  margin-right: -530px;
}

Fade-in share buttons.js

$('.post-sharing-side').hide();

var entryheight = $('.entry-content').height();

$(document).scroll(function () {
    var y = $(this).scrollTop();
    if (y > 350 && y < entryheight) {
         $('.post-sharing-side').fadeIn();
    } else {
        $('.post-sharing-side').fadeOut();
    }
});