The site itself isn't bad, in fact it's a pretty slick design, but there are a number of concerns here. First:
About the social media buttons, we know they cover content at around 960px width. But most people (in our region) don't browse the web on low resolution screens (like the 960) anymore.
What your analytics aren't telling you is that the majority of your visitors aren't browsing with the window maximized. In fact, most of them are seeing the buttons floating over the content. I'm not saying you should convert it to a full-on responsive site (though being a web design agency, you should definitely consider it), but you should at least fix that. The point is, the screen resolution on your analytics is the resolution of the monitor they're using, which is not a good indicator of how they're viewing your site.
Second, the site is a mess under the hood.
Exhibit A:
Code:
<script type="text/javascript" src="/media/js/jquery-1.4.4.min.js"></script>
<script type="text/javascript" src="/media/js/jquery.raptorize.1.0.js"></script>
<script type="text/javascript" src="/media/js/parallax.js"></script>
<script type="text/javascript" src="/media/js/cufon-yui.js"></script>
<script type="text/javascript" src="/media/js/boton-amtw.js"></script>
<script type="text/javascript" src="/media/js/jquery.scrollTo.js"></script>
<script type="text/javascript" src="/media/js/jquery.tooltip.min.js"></script>
<script type="text/javascript" src="/media/js/jquery.cycle.all.2.72.js"></script>
<script type="text/javascript" src="/media/js/jquery.ui.core.js"></script>
<script type="text/javascript" src="/media/js/jquery.ui.widget.js"></script>
<script type="text/javascript" src="/media/js/jquery.ui.mouse.js"></script>
<script type="text/javascript" src="/media/js/jquery.ui.sortable.js"></script>
<script type="text/javascript" src="/media/js/jquery.infieldlabel.min.js" charset="utf-8"></script>
<script type="text/javascript" src="/media/js/jquery.validate.js"></script>
<script type="text/javascript" src="/media/js/jquery.fancybox-1.3.4.pack.js"></script>
<script type="text/javascript" src="/media/js/preview.js"></script>
<script type="text/javascript" src="/media/js/base.js"></script>
<link href="/media/css/reset.css" rel="stylesheet" type="text/css" />
<link href="/media/css/text.css" rel="stylesheet" type="text/css" />
<link href="/media/css/fancybox.css" rel="stylesheet" type="text/css" media="screen" />
<link href="/media/css/nieuws.css" rel="stylesheet" type="text/css" />
<link href="/media/css/style.css" rel="stylesheet" type="text/css" />
That's a lot of JS, a lot of CSS, and a lot of server requests. Try to reconsider just what jQuery plugins you need, get rid of the ones you really don't, and concatenate/minify the rest. Same with the CSS.
Exhibit B:
From lines 295-301 of your source:
Code:
</div></form>
</div>
</div>
</div>
</div>
</div></div>
"Divitis" doesn't quite cover it. This is a full blown div outbreak. In 6 lines of code, you have 7 closing div tags. Cut down on divs you don't need. If you find yourself doing something like <div id="wrapper"> (or the Dutch equivalent), you're probably using too many divs. Try to cut out the ones you don't need and directly style elements instead.