Facebook has patented the ability to continue tracking users after they have left their website. Despite this Facebook repeatedly claimed that they were not in the business of tracking their users. However, Facebook's business is knowing their product as well as possible. You are their product. They are extremely interested in knowing everything they can about you. Why? It's really simple. The more they know about their user's online browsing activities the better they can customize ads for you. I imagine that they will create some pretty sophisticated models to determine who will click what sorts of ads. The more people click the more accurate the ad targeting will become.
While individual users do have a web "fingerprint" as the EFF puts it, people will typically browse the same types of websites together. For example people who play fantasy football will be going to yahoo! sport (or some other competing service), they then visit sites like espn, sports illustrated and probably a few sports blogs to try to figure out the best way to get an edge in their game this weekend. Facebook will take this data and aggregate it for a larger set of data. As there are 800 million facebook users and millions of players of fantasy sports, this data could be extremely useful for Facebook to use in placing ads. From these data they may be able to determine which sports team you're interested in, which players are on your fantasy team, and then display ads for jersey's from that team or for a specific player. They will also be able to figure out which ads will have an higher likelihood of someone with your browsing profile to click on.
Facebook will then be able to set a premium for ads that they do this with, or they will earn more money from the number of clicks a given ad gets. This of course is why Facebook has decided to collect this data. Some of it seems harmless enough. It's not that big of a deal that Facebook is getting my fantasy football information, why should I care? Well, you don't just use the internet for fantasy football, you use it for banking, shopping and a plethora of other activities. Do you know what data facebook is collecting? I certainly don't. From the patent it is unclear what protections they are providing on the data they are collection. It also doesn't say what data they will be collecting when you visit a third party site.
As a personal precaution I have started to use Facebook in a separate instance of Chrome using the Incognito function. This prevents my browsing history from being saved and deletes many cookies. I have also taken to deleting all my cookies every time I close my browser. I don't do it myself Chrome does it for me. Additionally, these settings are available for both Internet Explorer and Firefox. I suggest that you look into doing similar safety measures to prevent Facebook from getting information from you that you don't want them to have.
Finally, the other thing that isn't really discussed in many places that mention the ads, this data is also being provided to law enforcement agencies. Now of course there's the whole if you aren't doing anything wrong then you don't have to worry about anything. However, this worries me regardless because I'm losing my control over what information is going to the government and companies. I don't like that. Patents like this one and cookies that record our daily activities are changing our private life into our public life.