You don't have to be on a social network to be "datamined." Your Internet provider records every page you visit and many aggregate that info (supposedly anonymously) and sell it. Your credit cards and debit cards may do the same. Many cell phones leave a GPS location everywhere your phone goes. Product identification / theft protection systems can be read (scanned) by as much as a 100' distance. Stores connect what you buy to your payment method and if it's not cash they know what you're buying by your name. Even the data in the US Census is made public. In most places in the US your image is captured by dozens, if not hundreds, of cameras every day. The only country with more cameras than the US is Great Britain. In many places the cameras are capturing an image of your retina and comparing it to a database all without your permission or, in most cases, your knowledge. Remember when you had to give permission to be fingerprinted or arrested? Need I give more examples?
So, privacy is an illusion. And given that I fail to see why people are so freaked out about datamining. And Google appears, at least, to be more responsible in their use of their collected data than Facebook or Twitter or Yahoo.
I can understand being concerned about how the data will be used. If it's only to try to sell us things we don't need and may not want where's the problem? If the purpose is more nefarious, well...
Anyone who is seriously concerned with maintaining privacy (and complete privacy is total anonymity) needs to get completely off the radar. That means moving to a remote region (like northern Canada), destroying all your identification, living completely off the grid without a phone, internet, mail service or anything else that connects you to any other living person outside of your significant other.
Ridiculous, right? Yes, totally. Given that, IMO, being vigilant is the prudent thing to do and the only thing practical. Social networking has been so successful because we're social animals and although none of us need Facebook, Twitter, Google+ or any other one I would feel poorer without the ease of interactions with persons all over the world that I would not otherwise have.