Most people don't spend a lot of time thinking about the magic that takes place behind the scenes when they are on the internet. When it comes to advertising, tailored ads often lead people to assume that the advertisers know something about them and that often creeps them out. In most cases, it is not the advertiser who knows about you but the site you are visiting knows you. Or the personalisation is based on assumed demographics. Or information that you've provided to the company that serves the ads on behalf of others. It is seldom because the site you are visiting has sold your data to the advertisers.
For example, Facebook often serves me ads that seem to know about my age, marital status and some of my interests. The advertisers don't know any of that information about me, but Facebook does. If a business asks Facebook to present its ads to a particular group of users, such as handsome, young, brilliant privacy lawyers, I'll see that ad. The company that bought the ad will not learn anything about me unless I click the ad and decide to give them my personal information.
This great article, forwarded to me by a friend, explains a lot about the myths and realities of online advertising: Privacy MythBusters: No, Facebook Doesn’t Give Advertisers Your Data!:
It’s a myth that Facebook is hell-bent on getting users to share more information more widely for the sake of of advertisers. In fact, advertising on Facebook doesn’t involve sharing information about users with advertisers. In fact, advertisers buy ads that Facebook shows to users Facebook (or rather, its algorithms) thinks might be interested. If anything, sharing more information can actually help Facebook’s competitors if users take advantage of Facebook Connect’s data portability to port their data over to competing platforms. So the widely perceived conflict of interest between Facebook’s economic interests and users’ privacy just doesn’t exist. The site gains from having more users spend more time on the site, not from tricking users into “giving up their privacy.”