Privacy advocates are raising further concerns about the use of personal information from social media sites.

They say the practice known as "sentiment analysis" or "sentiment mining" may not be illegal but it is often unethical because people are unaware it is happening.

While there are obvious privacy concerns around the misuse of hard personal data - where you live, how much you earn, your medical records and so on - they say the use of soft data such as how you feel, what you like, your values - the personal sentiments that shape your behaviour - blurs the lines of privacy protection.

"I think a lot of consumers will be quite concerned that in reality what's happening on social networks is that there's a lot of marketing companies sitting over your shoulder," said Chris Connolly from the Australian Privacy Foundation.

"They're reviewing every comment that you make - positive negative, about brands, about products, about movies, services etc and they're collating all of this information and re-selling it to other organisations to make a profit.

"You're not getting any benefit from that particular sale, so I think there's a fine line between what consumers would allow that information to be used for."

Social media companies like Facebook release enormous amounts of raw data generated by users of their sites to companies like People Browsr.

"All of the networks have what they call a feed and various people in the industry are able to access that full feed of data," says People Browsr CEO Jodee Rich.

"Twitter makes it available, Facebook is now making it much more available, MySpace is making it available for three months, in the blog world it's been available for some time.

"When you put all that data together you're now getting a velocity of about 5,000 new posts every second, and that is laying down a historical framework of tweets, of posts that we can go back and look at.

"Many of those Tweets or posts contain interesting information about how people are feeling - whether it be about themselves, their country, their day, their wife, their husband, or the brands that they're interacting with."

Advertising strategies targeted at users of social media are developed by analysing these personal sentiments.

Advertisers argue it is helpful to consumers.

"To have products and services that are targeted more towards the sort of things that I'm interested in is actually useful to me, rather than having random adverts that have nothing to do with me," says Marie Claire Jenkins, a digital strategist with McCann Erickson.

"So I think consumers need to think about it as a personalised service that helps everybody."

The targeting can also be unethical, says John Clippinger, co-director of the Law Lab at Harvard University.

"You look at the fact that Google can mine Gmail and send you offers, and you may have a discussion in an email about your doctor, about cancer and suddenly you're getting spam from a cancer drug or something," he said.

"These are things that really should not be encouraged."

And Mr Clippinger says the targeting will intensify with the growing use of smart phones connected to the internet.

"The phone can know who you're interacting with, when, what time and at what location. That can be very powerfully predictive and create a signature or a unique pattern as to who you are and your likelihood of buying and doing certain things," he said.

Background Briefing, Radio National, 9.10am Sunday June 27 (repeated 7.10pm Tuesday June 29)