Okay, so here's a little explanation on why some folks are a little bothered by Facebook data mining. It may still very well not bother you. You may not be participating in all of these collection streams. Thats fine. But maybe you will gain some understanding. This is not a complete list. I just banged this out in a couple of minutes. If we sat and hashed it out, we could have a much more complete list of risks.
* One need only read a little bit of the various security expert's opinions on the "Snowden documents" to understand the relevance of metadata and the huge amount of data you can glean from someone just from metadata. In the case of FB, we're talking much more than metadata. We have detailed data on users: Name, sex, sexual preference, political preference, location, whom they are associating with, whom they are related to, whom they are currently with, religion, financial status ... the list goes on.
* Unless you have explicitly blocked FB's cross-site tracking ... or use a totally different browser instance for FB, they pretty much have tracking to determine exactly what web sites each user visits.
* Some (possibly all) of that data is shared with FB advertisers.
* If you use FB's built in app platform, this data is potentially shared with application developers -- who may again share this with their affiliates. FB apps essentially log in AS YOU and can pretty much do what they want AS YOU. Your privacy settings really don't apply.
* If you use FB apps on your phone, they will have location data... not just for check-ins, but for pretty much all the time.
* Government agencies have a great interest in FB data. Government has requested of FB a huge number of national security demands. We don't really know what they've requested, just very general numbers. There is a huge undercurrent in IT industries to create data mining partnerships with governments. What is being requested and complied with is generally just not known. This mostly applies to "US government" ... but it also applies to others... As the data golden egg grows, more and more will participate.
* Advertising is not necessarily as simple as you think it is. It isn't necessarily just an image saying "Buy widgets!" They are usually javascript that runs on your computer. They are not designed by Facebook, but by some third party. This might be someone with very little security coding knowledge that makes dumb mistakes. It could also be a bad actor (malware, etc). A huge amount of malware propagates via ad-based injection. Now... with good data the malware can be targeted to a single person... or group (gun owners, people that have a particular stance on abortion, people likely to have bigger bank accounts, etc).
* If you trust FB today... it could be sold tomorrow. The holders of your data can and will change over time. Deleting information is of little use. They have it.
* Many sites are starting to use FB as a single sign on entity. This is A REALLY BAD IDEA. This means putting all your eggs in one basket. If it gets compromised/hacked/etc... you get EVERYTHING that is attached to it.
* There is a significant risk of hackers getting FB data. They already glean lots of single accounts and sell them on the open market. Getting the backend data would be worth quite a lot of money to them. FB is a pretty decent target.
* The primary concept of computer and network security is "least required privilege". FB design is exactly opposite of this. It's just bad design that is sort of set up for major security issues. It may never happen. It may happen every day and not be published. But trust me as a "computer security professional" ... It is a "bad thing."
Once again: I'm not listing all this to be all tinfoil hat paranoid. Computer paranoia was my job for over 25 years. If you are comfortable participating and sharing fully, please continue to do so. If you think you're not sharing much... you are likely wrong. Learn what's likely being shared and decide if you're still comfortable.