Details could be leaked, said one Lib Dem MP
Social networking sites like Facebook could be monitored by the UK government under proposals to make them keep details of users' contacts.
The Home Office said it was needed to tackle crime gangs and terrorists who might use the sites, but said it would not keep the content of conversations.
It is part of a plan to store details of all phone calls, e-mails and websites visited on a central database.
Civil liberties campaigners have called the proposals a "snoopers' charter".
Tens of millions of people use sites like Facebook, Bebo and MySpace to chat with friends, but ministers say they have no interest in the content of discussions - just who people have been talking to.
Liberal Democrat MP Tom Brake said the websites contained sensitive personal details and he was concerned information could leak from any government-controlled database.
The Independent newspaper quoted him as saying similar plans to store phone and email records threatened to be the "most expensive snooper's charter in history".
"It is deeply worrying that they now intend to monitor social networking sites which contain very sensitive data like sexual orientation, religious beliefs and political views," he said.
The newspaper also reported that Chris Kelly, Facebook's chief privacy officer, was considering lobbying ministers over the proposal, which he described as "overkill".
Phone companies are already required to store details of all calls, such as the time and date, location and who made them, for 12 months for possible use in criminal investigations or court cases.
An EU directive ordering data on internet traffic to be stored in a similar way is due to come into effect in the UK on Monday, 6 April.
The government is also considering proposals to store all communications data on a single database, which may be run by a private company.
It has delayed legislation on the move amid concerns about civil liberties and is due to launch a consultation on the plan "shortly", which will set out privacy safeguards.
The Home Office claims the new database is necessary to allow police and security services "keep up with technological advances" and that billing information is already stored by telecoms companies.
A spokesman said: "The government has no interest in the content of people's social network sites and this is not going to be part of our upcoming consultation.
"We have been clear that the communications revolution has been rapid in this country and the way in which we collect communications data needs to change, so that law enforcement agencies can maintain their ability to tackle terrorism and gather evidence."
Shami Chakrabarti, of campaign group Liberty, said she would be "flabbergasted" if the the police and security services were not monitoring social networking sites already and it was "permissible" on human rights grounds to examine the profile of suspects.
But what she said was unacceptable was the government storing all communications data centrally, which she said would allow them to monitor the web browsing habits of ordinary citizens.
"With websites, as opposed to traditional phone calls and e-mails and so on, the difference between what the website you're visiting and what you're doing there, is really blurred.
"I mean just by my web browsing habits, just by which sites I'm visiting, you'll be able to build up... a pretty detailed picture of who I'm associated with, perhaps what my politics is, what my religious preference is and shopping habits are.
"It's a pretty detailed bit of surveillance about a person, about all individual people, most of whom, let's be clear about it, are completely innocent."
She added: "That's the difference between being a suspect and just an ordinary citizen, being part of the mainstream population and going about your business in a normal way."
Details of the social website proposals were disclosed by Home Office minister Vernon Coaker earlier this month, at a Commons committee to examine draft EU directives.
He said that the government was considering acting on social networking sites because they were not covered by the latest proposals from Brussels.
Mr Coaker acknowledged that the plan would raise fresh concerns about the right to privacy, saying he accepted it was an "extremely difficult area".
"It is absolutely right to point out the difficulty of ensuring that we maintain a capability and a capacity to deal with crime and issues of national security, and where that butts up against issues of privacy," he said.
The Cabinet Office already monitors popular social network sites such as Facebook, Netmums, Fixmystreet and Mumsnet to see what users are saying about public services.
That's just what we need. The government having access to all this information. They have proven with alarming consistency that they cannot be trusted with sensitive information. Who knows whose hands these data will end up in when some bright spark leaves a hard drive on a train.
That the government is considering encroaching even further into people's private lives is horrifying, although hardly surprising. The move is yet another indicator of the government's seeming desire to spy on its own people, and the idea that criminal gangs would barefacedly announce what they're doing on a social networking site is somewhat unbelievable. It would definitely make me think twice before joining.
Harry, Nottingham, UK
I've got nothing to hide - go ahead! And if you do have highly personal information about yourself that you would like kept sectret, why is it on a social networking site in the first place? They are not secure.
Amy Baker, Adelaide, Australia (but English)
Any terrorist or criminal that currently uses social networking sites to plan their crimes (a scenario I find highly unlikely) will just stop using it. This will leave the innocent users under surveillance for no justifiable reason. The government is obsessed with monitoring and controlling the population. General Election soon, please
I work for the Police and I for one think this is a fantastic idea along with every other scheme that is or is threatened to be brought in ot fight this insidiuos and invisible fight against terrorism. I can't wait to change my title from Constable to Stasi
Robert Pangborn, Surrey
Ah, the inexorable rise of the security services. Ask a security expert what's needed and, guess what, their recommendation is 'more security'. How many people were killed by terrorists in the UK last year? Or the year before? Every loss of freedom is a massive victory for the enemy. The biggest threat to our way of life is actually our government and its agencies, scaring us into compliance and acceptance of hitherto unimaginable invasions of privacy. People, wake up.
Robert Simmons, Dorking, Surrey