Privacy

Anecdotally, yes. I know that the similarities in profile and avatar photos as well as signature lines on web forums have been used as evidence to link digital profiles together. This linkage then allows warrant level evidence to be used to subpoena the other sites for all user details. Another one that is much harder to hide is writing style. People have a relatively unique way of writing that is often enough to link data. Even when the user name, ip addresses, and everything else is different.