Collusion is an plugin for Firefox and Chrome browsers....
You may not realize who collects what, which is exactly what Collusion wants to make clear. For instance, you do not need to be registered with Facebook for them to make a profile of you. Once you've visited any page that is affiliated with them, they'll create a file about you and collect each and every visit to every site that has a "Like" button or a Facebook plugin. Google is even more extreme, as they collect data from every place that has AdSense, Analytics, and similar services, which basically covers almost everything the average person visits. Those services may not always be as obvious as a "Like" button - for instance, some are implemented by displaying a single transparent pixel image.
You may not realize in which way this data can be combined, and how complete these profiles actually are, apart from just showing what news you read. You cannot know what kind of surveillance methods and laws will be implemented in the future. Already, biometric information gathering such as the identification of people from video recordings is becoming more and more successful, even prompting for the EU to begin implementing a system that can link people in public places to their Facebook pages and other photographs. Similar plans are implemented by the US. Other technologies include public voice surveillance, supervision of vehicle movement or behavioral analysis in public spaces. All this data can and will be linked and combined with what is collected about you online.
This kind of information isn't something that will stay in one place forever and will only be used for one purpose. This may be as trivial as the ToS changes over at Instagram, where people would surrender the copyright to their photos for Instagram to use, and it may be as severe as a fundamental change in government that asks for access to this data. Again, this may be as tolerable as FBI warrantless surveillance and as extreme as a dictatorship craving information about previously acceptable behavior.
I'm not trying to evoke Godwin's Law, but what would have happened if the Nazis had access to all communication data of their citizens, allowing them to see who had ever communicated with a Jew, and what they had talked about? These suspicions may seem outlandish, but who would have considered it realistic for Greece to have a sizable Fascist movement ten years ago? And this information will not only be around for ten years, but possibly until the end of your life, and it could still affect your offspring afterwards.
It is a central premise of computer technology that no system is ever completely safe. This means that your information can fall into the wrong hands by illegal methods, possibly by technologies we can not yet imagine. There may be huge obstacles to overcome in order to raid Facebook's or Google's servers, but there are myriads of tracking services available which the average user will not scan for security problems or loopholes in the ToS. If one of these is compromised, your information is out in the open to be transmitted without your control or judicial supervision.
There are a lot of psychological and sociological implications to constant and ubiquitous surveillance. It has been proven in numerous experiments that people will behave very differently if they know that they are being watched, especially if they do not know if there's a supervisor currently present and who they are. This applies to only to a limited extent to online behavior right now, because few people realize the amount of surveillance which is already happening.
It is not a logical fallacy to assume a slippery slope in this case, because as the change happens slowly, people grow accustomed to it. I was ridiculed years ago for outlining some of the things we currently see, but when I remind people of that nowadays, they respond with "so what". For that reason, we have to be aware that these changes may lead down a very dark road.
As an example, I vehemently oppose the usage of mobile surveillance drones, while a press statement by our police presidents says that the upcoming models will "not yet be armed due to insufficiently evolved technology". None of my peers have expressed outrage at this concept. This is potentially very harmful to society, and every bit of surveillance furthers the mindset to accept such laws.
Those are main reasons for opposing surveillance. They apply to any kind of surveillance, of course, but tracking Internet usage is a central part of that in today's massively networked world.
If you're interested in other perspectives, there are lots of articles available online. Following the publications of the Electronic Frontier Foundation would be a good starting point for a better look at the sociological implications, and seeing TED talks by technology activists such as the Chaos Computer Club or the participants of DEFCON would be a good way to learn more about the technological side.
