Ever wonder why things like Google, Facebook and Twitter are free? How exactly does a service like that provide so many useful features, and continue to develop more on a daily basis, while providing all of these things for absolutely free to millions of users?
Wait, you didn't think you weren't actually paying for these things did you? Did you think you owned the content you posted and that it wasn't being sold by those sites? Were you not expecting to find tweets which you don't even have access to anymore being used to line twitter's pockets?
It's not a matter of privacy, it's a matter of business. You're willing to give up all rights to anything you tweet, search for on google, or post to Facebook.
Yes, these companies make their sites run through advertising agencies, and any modern advertising agency is concerned with how to better reach out to their consumers. It's called Targeted advertising, and it's not something exclusive to these sites.
I don't understand why everyone is suddenly surprised when these things happen. If you're posting to social media sites, you're giving up all rights to that data, you're saying "Yes, I'm ok with you knowing this information about me, and even selling it to other companies, but that's what I'm willing to give up in order to use this otherwise free service".
You are not the customer of Facebook, Twitter, or Google, The customer is the advertising companies. You are simply the product being sold.
Keep the internet free; don't remove your online footprint or suddenly stop tweeting, this is how things are if you want things to be "free".