Oh, BlueKai is only the tip of the iceberg. I used to work for a spammer (that called itself an "incubator") that used blukai among other better sources. 3rd p

Oh, BlueKai is only the tip of the iceberg. I used to work for a spammer (that called itself an "incubator") that used blukai among other better sources. 3rd party cookie impressions can be sent to a company like LiveRamp to identify what websites users have visited (in theory only in buckets, but that's trivial to bypass). With the exception of the very largest websites, you could see traffic to many, many websites and tie it back to an individual user (even if their IP changed, etc.). Companies like FullContact and People Data Labs offer the ability to take the few pieces of information you have a user and get their other info (eg; verify email using name and address, or get social media profiles, etc.). The problem is no matter how good you are about using VPNs, clearing cookies, etc. the weak link are the websites you're forced to give some info to. Many of them will well you out even if their TOS say otherwise.