The persistent "retargeting" takes advantage of tracking technology and is now a strategy for several companies in their campaigns that understand a first encounter with a product isn't necessarily the point of decision on a sale.
The relevant ads aren't merely related to categories users have followed. They're personalized to the point of serving ads about products or services someone has at one point perused. They follow someone around.
The response has been generally positive, the Times reports, although some feel stalked by products they didn't particularly want but had evaluated --- or were sharing a computer and didn't want others to know about what they were perusing.
Critics are also wondering about whether privacy is being breached and regulation needs to be introduced to moderate the phenomenon.