“.#[random-char-string]” being inserted at end of URLs

I’ve seen this on a couple of sites now and I’m having trouble finding information about (probably because it’s awkward to describe):

All internal URLs like this for example:

    http://www.domain.com/section/

Are rewriting themselves to something like this:

    http://www.domain.com/section/#.TvuV_pim9PI

Does anyone know what causes this? I’m running WP 3.3 and .htaccess is just the default:

    # BEGIN WordPress
    <IfModule mod_rewrite.c>
    RewriteEngine On
    RewriteBase /
    RewriteRule ^index.php$ - [L]
    RewriteCond %{REQUEST_FILENAME} !-f
    RewriteCond %{REQUEST_FILENAME} !-d
    RewriteRule . /index.php [L]
    </IfModule>
    # END WordPress

My Permalink settings are set to Post name and nothing in Category and Tag base.