Some marketers say that early results are promising but that advertising on Twitter remains an experiment. Other marketers, including PepsiCo Inc.'s beverage brands and Best Buy Co., who tested out Twitter's new advertising products—some without cost—haven't made new ad buys. Marketers say they definitely aren't ruling out advertising on Twitter in the future, but that it's still early days and they are figuring out what works.
That kind of data is where the really big bucks are, and Twitter is just a beginner in the race. It's why we've seen Facebook being so casual with privacy.
If any company doesn't explicitly guarantee you a certain kind of data privacy, then they can and often will monetize the rest of the data that you expose to them. Many ISPs sell clickstream data to many takers.
The practice not even close to new. Offline it's been going on for decades with catalogs and other kinds of direct marketing.