The social networking website ask.fm is at the centre of a storm in Britain following the suicide of a schoolgirl who was bullied online -- with advertisers pulling out and the prime minister leading calls for a boycott.
Hannah Smith, 14, hanged herself last week. Friends said she had been taunted on ask.fm about everything from her weight to the death of her uncle. Bullies bombarded her with anonymous messages, telling her to "drink bleach" and "go die".
Campaigners say she is not the first teenager to be "bullied to death" on ask.fm, which counts millions of adolescents around the globe amongst its 60 million registered users.
The Latvia-based website -- which has turned its founders, brothers Mark and Ilja Terebin, into millionaires since it launched in 2010 -- has been linked to at least four other teen suicides in Britain, Ireland and the US this year.
Hannah's death has pushed criticism of the website to fever pitch, with parents -- including Prime Minister David Cameron -- asking why users are able to post such vicious remarks with complete anonymity, often to youngsters in the same school class.
Wary of the negative publicity, advertisers including telecoms giant Vodafone and designer Laura Ashley have scrambled to abandon the site -- and after Cameron called Thursday for users to boycott it, ask.fm's founders were forced to issue a defence.
"We are committed to ensuring that our site is a safe environment," the Terebin brothers insisted in a statement. "We do not condone bullying of any kind."
The website had already described Hannah's suicide as a "true tragedy" and said it is working with police investigating the death.
Her father has called for its operators to face murder or manslaughter charges.
-- 'No accountability' --
Ask.fm has a question-and-answer format -- users pose each other questions, such as "Who is your best friend?" and their answers are posted on their profiles.
Crucially, users can choose to post anonymously -- a feature that campaigners say has made the website a playground for cyber-bullies.
"People can go on to the website and produce pure hatred without being identified," said Anthony Smythe, managing director of the charity BeatBullying.
"There is no accountability."
Charron Pugsley-Hill agrees. Her niece Ciara Pugsley, 15, was found hanged in the woods next to her house in Ireland last September after she was bullied on ask.fm.
"The anonymous nature means people will say things that they wouldn't say in real life," said the 48-year-old artist.
"That gives rise to paranoia -- who do you trust? It could be the person you sit next to at school."
Ciara was a "feisty, very bright" girl who had represented Ireland at karate, Pugsley-Hill told AFP.
"This was not a girl that sat quietly in the corner," she said. "If it could happen to Ciara, it could happen to anybody."
Ask.fm has defended the anonymity feature, saying its users can choose not to receive questions from anonymous users.
In extreme circumstances -- such as the suicide of a teenager bullied on the site -- it says it can trace the source of an anonymous post and pass the information on to police.
For many British teens, dealing with online abuse has become almost a part of daily life. One in three young people are cyber-bullied, according to BeatBullying.
Andrew, a 14-year-old from Scotland who uses ask.fm to share his views on everything from his love life to TV shows, said he had publicised his posts on Facebook -- only to be met with suggestions that he should go and kill himself.
"I must have known the people who were doing it," said the teenager, who checks his ask.fm page every few hours for new posts.
Like many others, he has simply learned to take the abuse in his stride.
"It didn't bother me. I just deleted it," he told AFP.
Some users relish the honesty that ask.fm's anonymity feature brings.
"I use it because it can give you an insight on what people think of you and no one is fake there," another teenager told AFP.
Internet abuse has been top of Britain's news agenda in recent weeks after several high-profile women, including two lawmakers, received rape and death threats on Twitter.
Three men have been arrested over the threats, while tens of thousands of people have signed petitions calling for Twitter and other websites to tighten security.