as Ajedi32 states, even knowing entrances and exits, it is SUPRHARD to figure out all the associations, but it has been done in the past for high profile sites. It is also not possible for them to monitor every entrance and exit, and it is akin to watching the entrances to a mall where everyone is dressed exactly the same, trying to identify who shops where. With one-off data, really hard. With regular or periodic data, the mystery is a lot easier to unravel.