Here is a screenshot from the self-test I did with the de-anonymizer described in my last post. I'm a member in 5 groups at Xing, but only active in just 2 of them. This is already enough to successfully de-anonymize me, at least if I use the Google Chrome Browser. Using Microsoft Internet Explorer did not lead to a result, as the default security settings (I use them in both browsers) seem to be stronger. That's weird!