It is easy to collect data about who is doing what online.
Google collects data about all your searches; it masquerades as a service, storing your search history for example, but it gives them a huge database of user activity on which to base their product development.
Others are collecting data so that they can serve ads that are targeted to your online profile. Facebook have recently faced a community backlash for it and have revised their policy (Guardian article). Ad networks are aggregating data from their sites so that they can extend their targeted inventory, re-inventing their run-of-site inventory as behaviour targeted. And now ISPs are collecting behavioural data (Clickz article).
But why are they doing it? It is all about adding a premium to the available advertising inventory. For example, in the past if an advertiser wanted people who were interested in cars, they would have to buy inventory from a car site. Now they by ads that are on other sites, but only when they are being looked at by someone who has recently been on a car site. This is known as behavioural targeting.
We know the upside for the companies collecting the data; they are making more money. But what does it mean for you. On the one hand, the Facebook backlash people would say that it is infringing their Data Protection rights. But on the other, as represented by those profiting from the activity, it means that you get ads that are more relevant to you which is a useful service. Imagine a magazine where every advert you saw was for something you'd been thinking about buying! There would be plenty of people who'd pay for that, especially if it included some good offers.
Like it or loath it, it will happen more and more. There is evidence that the youth culture is embracing it and accepts it as a part of live. Do you?