I saw an interesting Youtube video about the connections between the creators of Facebook and the CIA and other governmental organizations; the point of the video was very Big Brother -esq.
That being said, everyone commenting here is forever cast on a public website where there are certainly spies....
I have a FB account and display parts of my life that I want to put forward that I also feel won't be of detriment to me. I don't typically list my location, phone numbers, email address, address, or private information. However, I think it is also good for me, as a gun owner, to share my positive gun experiences so that all can see that gun ownership is a healthy natural thing.
I do think that there is significant data mining on FB, and everywhere else for that matter. Not much data to mine from me, frankly. I have basic information and pictures... I don't play the games and I don't share much else.
I also don't wear the tinfoil hat. I very much live on the grid. I have credit cards, which track where I spend money and travel. I have guns, many of which required FFL forms, which show what I own and where I live. I have a professional license, which requires that I provide my address. I have been fingerprinted for my career no fewer than 3 times. I have been DNA sampled and blood typed. My medical records are contained on a government database. My home address is contained on a handful of private and public record databases. Etc. Etc. Etc.
Folks, in the modern world if you want to exist you pretty much have to play by the rules of living life on the grid.
Take measures to protect yourself by not giving more information than necessary to function, watch your surroundings, guard yourself and possessions, and use common sense.