Bugs on web pages, software that sends data to its manufacturer's web site, and everywhere you look - advertising. And sometimes, it seems like this advertising knows that it's you who's looking at it. Before computers were commonplace, companies had to rely on focus-groups, questionnaires, and myriad other inefficient methods for gathering demographic data. When asked why they wanted this information, the answer was typically to the effect that it would help them better target their products to their customers or it would help them find their customers. That was saved for advertising, where warm fuzzy emotions are combined with flattering images of a product, more to confuse the issue than to help anyone besides the company selling the product.
Deceptive tactics are nothing new to advertising.
How about getting a pile of unsolicited catalogs after mail-ordering one item. It seems that as far as advertisers are concerned, any single thing you do can be considered a representation of your overall habits. Unknowingly click a link that takes you to a porn site, and for the next few weeks you might see more porn ads in those banners. I wonder how that could be. With so many people online these days, and so much information available, advertisers and software development companies have found ways to acquire a wide range of information and to quickly reduce it to a profile.