With only a few notable exceptions, humans are inherently more social than solitary. People are openly sharing their lives online with friends, family and even strangers via Facebook, Twitter, Foursquare and more; in short, we all want to share who we are and what we've achieved -- and doing so online is the most effective way to accomplish this. Many even go so far as to openly broadcast where they are physically standing and what they are doing (or eating, or watching or listening to). There are, of course, people who feel that the drawbacks to this rampant sharing outweigh the benefits. I recall a friend of mine recently recommending that I stop using the popular TripIt app because I was broadcasting to potential thieves when I was home and when I was away.
There is a parallel in advertising -- advertisers are grateful for all this sharing and use it to make sure that they are promoting their products to people who are actually interested in them and not wasting money on those who are not (e.g. promoting diapers to childless men.) As this has become more obvious, for example, those red shoes you considered on an e-commerce site that now seem to follow you from website to website, some have said this is invasive, annoying or even creepy.
Despite this, advertisers will continue to target their ads to reach the right audience, and eventually these ads will no longer be considered "creepy" for several reasons.
Customized ads are actually helpful. Though probably not for the reasons you may think. Consumers, and even people within the advertising industry, actually cares little about what ad is placed in front of them, as long as it's useful. You're more likely to pay attention to an ad if it is relevant to you and your interests, but, if it's not, we are all very capable of ignoring it. And there's the rub: advertisers want to reach the right audience and are prepared to pay more for the privilege of doing so. The better the advertisement works, the more products are sold and the more money is spent. And in most cases (when the content is free) it's the advertising dollars that are funding the content creation. Funnily enough, it's just like a social network: the more people use it and share information openly, the more engaging and interesting the entire ecosystem of publishers, consumers and advertisers becomes.
Back to the TripIt example. I still use the app, despite my friend's warning, because I've grown to value the random call from a long-lost friend who happens to see that I'm traveling to town for a day or two. For me, the connection that this app affords me outweighs any perceived risks.
The silver lining in all of this is that believe it or not, the disappearance of anonymity is a much more natural lifestyle for humans. We evolved to live in small groups where people are social and know things about one another. Yes, the online space has grown by leaps and bounds over the past 10 years and will continue to grow -- but as we become increasingly connected via online channels, one might argue that it's simply digital evolution at its finest.