Spam Links Appearing

Hi,

Recently I have noticed some spam links appearing at the top of all the pages of my website http://www.repscience.net

The spam link is <a href="http://barberforcommissioner.com/buy-viagra-without-a-prescription-in-taiwan/">buy viagra without a prescription in taiwan</a>

The link is being inserted into the div <div id="fb-root"></div>

I have performed a quick fix by changing the above code to <div style="display: none;" id="fb-root"> so it doesn’t show to visitors, but it’s still there in the source code.

I have done an entire site search and database search for the link, but I can’t find it anywhere.

Has anyone else seen this issue? Anyone know how I should fix this?

Help very much appreciated, Thanks.