I've seen this technique used, in conjunction with popular sites that skew heavily male or female, to guess the gender of the user for ad targeting. I've also

I've seen this technique used, in conjunction with popular sites that skew heavily male or female, to guess the gender of the user for ad targeting. I've also seen this technique used on software-as-a-service sites to check for visits to competitors. If the user's already visited a competitor's landing page, an us vs. them feature comparison is shown. If the user's visited a competitor's page that's only accessed by their paying customers, offer them a coupon for switching. If the user has never visited a competitor at all, don't mention any - why educate the user about alternatives? Using history sniffing to validate the quality of data purchased from BlueKai is a new one to me, but plausible. Whatever the rationale, it's still a violation of user privacy.