Data mining is done on all aspects of internet use, social networking sites, such as facebook, are where data mining turns into data strip mining.
Being wary of giving out a lot of personal thoughts or information on these sites is healthy, what information you put out now, may not be flagged or considered subversive, (eg: telling the truth), but tomorrow who knows what will be considered subversive, dangerous etc, to the government or PTB.
I don't belong to any social networking sites, for precisly this reason, and as these sites are becoming the first port of call for data miners.
Plus, all the people I know personally that spend a lot of time on these sites are really annoying :)