Sometimes the "spies" are unexpected.
Users, often teenagers or 20-somethings, have been caught short by university officials and future employers who have used the networking site to check up on a person's behavior.
Oxford University student Alex Hill told the Guardian newspaper in July that she had been charged by university authorities "for breaching rules and being disorderly' on the basis of photographic evidence from Facebook."
The student union has now advised students to tighten restrictions on their Facebook profiles, although its president, Martin McCluskey, said that many people continue to allow access to everyone from the Oxford University network - which includes tens of thousands of people.
For some users, the privacy stakes are even higher - a political storm broke out last month when the media reported that the 17-year-old daughter of US Republican presidential hopeful Rudy Giuliani had joined a Facebook group in support of a Democratic rival candidate Barack Obama. She had to take down her profile.
"We are really heading into new modes of publishing ourselves, we are kind of learning on the job," university lecturer Dwyer said. "People trust their own judgment."
Charlotte MacCormack, a 17-year-old Facebook user from London, which now has the site's biggest network, conceded that it was "hard to be sure" who could access her details online.
"On the other hand, if I were that worried about it, I wouldn't be on here," she said.
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