Barbara Weed's son Tom, 18, cut himself off from his family after getting drawn into a controversial online community. She has not seen him since. Kate Hilpern reports

One Wednesday afternoon in May, when Barbara Weed's 18-year-old son, Tom, was right in the middle of his A-levels, he abruptly left home. "Dear Family," said the note he left on the doormat. "I need to take an indefinite amount of time away from the family, so I've moved in with a friend. Please do not contact me. Tom."

He has not been in touch with any of his relatives since. But Tom is not a missing person. His family know roughly where he is. It's just that he won't talk to them and they suspect he never will. "He got hooked in by an online cult," Barbara says. "The website convinces vulnerable people that they should hate their parents and should leave their family."

Even the wording of Tom's letter is from the website. Its founder says, "The letter should buy you six to 12 months before your family come looking for you and that will give you time to get used to living without them."

Barbara did not wait that long. "I tried to respect Tom's wishes and leave him alone, but once I discovered that the website was responsible for Tom leaving, I visited him at a cafe where he was working part-time," she says. She worked out that if she ordered a cup of tea, he would have to listen to her for about a minute. She told him that if he ever wanted to come home, he could. "He just looked at me, shaking his head, as if to say, 'You fool.'" What baffled Barbara was how a website could have such a dramatic effect on an ordinary family and in such a short space of time.

Barbara and her husband already had two sons - Nick, two, and John, four - when their youngest, Tom, was born. "I adored Tom," says Barbara. "Nick was the mischievous one and sometimes I did get cross with him. But I didn't need to get cross with Tom. He was such a joy to be with and had long, serious conversations with everyone. I always thought he would be the last one to leave home - that at 40 he might even still be here, which is ironic."

The boys were so close in age that they all played together. Barbara took them to parks, playgrounds, theatre shows and Alton Towers and, even though money was tight, there were family seaside holidays every year. "When Tom left, John said, 'But we had a great childhood.'"

By the time the boys reached adolescence, family life had become more dispersed. "My sons each had computers in their rooms and we all had such different schedules that family mealtimes didn't happen any more. Also, Tom was vegan and wanted to cook for himself, so I just gave him money to buy food and he just got on with it." By September 2007, his brothers were at university and Tom had found a girlfriend.

"It's as if you wake up one morning when your children are teenagers and you realise that practically the only time you talk to them is when they're going to the fridge," says Barbara. "But then there are moments when they do things, like give you a necklace." The necklace she is wearing was given to her by Tom after a summer holiday.

Tom and his girlfriend, meanwhile, had become increasingly interested in an online community called Freedomain Radio (FDR), which invites discussion around philosophy, politics and personal freedom. Unbeknown to Barbara at that time, a key topic of the site - whose members seem to be mostly in their teens and twenties - is the idea that ultimate personal freedom can be gained by cutting yourself off from any involuntary relationships (ie your family) and entering into completely voluntary ones (ie your new mates online). "I think once you get these corrupt people out of your life you will for sure have enough room for all the new awesome, virtuous friends in the world," said one member to another recently.

For members unsure about such drastic measures, there are podcasts with titles such as "But my parents were really nice!" and there is a chatroom in which members discuss how so many families are unjust. There is also a Sunday call-in show in which the website's founder counsels callers. Often the subject is leaving your family.

Barbara recalls Tom and his girlfriend looking forward to the Sunday call-ins and spending more and more time on the FDR website. "Tom did mention it at the time - although not their ideas about family - and I can remember alarm bells ringing when he said the man who ran it was giving him advice. I warned him that on the internet you don't know who you are talking to." By November 2007, Tom's behaviour had noticeably changed. "He wasn't spending time in his room just because he wanted to be with his girlfriend or on the computer, but because he didn't want to be with us.

"One night he blurted out that when he left home he wouldn't come back and that I would never see him again. At first I thought he was talking about university - that he wasn't coming home after that. But I was puzzled by the bit about never seeing him again. He responded that we had no relationship and that it was over."

Barbara says she tried everything - persuasion, negotiation, compromise. "But Tom didn't seem interested in communicating, merely in throwing accusations - for instance that his brother John and me were fond of laughing at him, which wasn't true. I began to notice that he was interpreting all family interactions as abusive.

"We did our best to be a happy family. Knowing what I do now about the website, I think Tom was being convinced by the online community that he had been cheated because he didn't have a perfect family upbringing. But who does? We really did try our best."

Then one day in May this year, when Barbara got in from work, Tom had gone. She read the note and was devastated. For a moment, she wondered if he had run away with his girlfriend (who has also since cut off her family to devote herself to FDR), but she and Tom had recently split up.

Then Barbara thought of the website and began to investigate. She quickly found references to something called "deFOO" - the name the website gives to ridding yourself of your "family of origin" (FOO). Then she came across Tom's thoughts posted on the site during the months leading up to his own decision to deFOO. Trying to think practically, Barbara topped up Tom's mobile phone ("I was worried he hadn't even organised where he was going to live") and the following day she phoned Tom's school, fearful of him quitting his education as hastily as he had family life. The school said he seemed fine. The next week, Tom's brother Nick waited at Tom's school all day to see him, but Tom wouldn't talk to him.

Another week passed; there was the exchange at the cafe and besides catching a glimpse of him at a local festival, and once on his bike, Barbara has not seen her son since.

"In the early days, I burst into tears all the time," she says, but now that some time has passed, she tries to keep things in perspective. "He could be floating down a river dead, but he's not. He could be somewhere that I don't know about, never sure if he's alive or dead, but he's not. I have to keep reminding myself that, as far as I know, he's well and happy."

In some ways, Barbara feels relieved that he has left Leamington Spa - where she lives and where Tom was born and brought up - and gone away to university. "I was dreading it, but it is so hard knowing I could see him any time. Also, I know he is starting his new life now. Every parent wants their child to be happy, to do well, and that's what he'll be doing. So that's great. I just wish I could be part of that - that I could give him another £50 when his student loan runs out, that I could celebrate his successes with him."

The Cult Information Centre - which says that several people have been in contact recently about family members recruited into cult-like organisations via chatrooms or other online means - recommends that families try to keep up some form of contact. "So I sent Tom a text message to wish him luck at university and tell him I'm thinking of him," says Barbara. "I don't know if he would have read the message or just deleted it when he saw it was from me."

Because Tom's new "family" is online, Barbara has - at least until recently - been able to see what he is up to. It's how she knew what A-level grades he got and it's how she knows where he's at university. "I spend far too much time on the site," she admits, logging on as soon as she gets in from work and often not switching off her computer until the early hours. "It's a bit like he's sitting at the next table. I hear everything he's saying, but I'm not in the conversation."

This month, however, the chatroom has been restricted to members only. "I can't go in as a visitor any more," she says. "I've lost the only remaining glimpse I had of him. I don't know how he's feeling or if he needs help."

Stefan Molyneux, the founder of FDR who attracts many people to his website through YouTube, tells me that he simply reminds people "that our family relationships are voluntary and you should really work, if you're unhappy in these relationships, to improve the quality of those relationships - but to remember they do remain voluntary. And that gives people the motivation, I think, to try to improve them. But if you can't improve them - and we can't change other people, as we all know - for sure you should have the option to disengage."

Molyneux, a 42-year-old former actor and IT worker assures me that what he calls deFOO is "actually quite rare". And although he and his wife (both of whom have deFOOd) are expecting a baby in December, he says on the website: "Deep down I do not believe that there are any really good parents out there - the same way that I do not believe there were any really good doctors in the 10th century."

Molyneux, whose Canadian home also hosts member get-togethers, brings up the word cult before I do. "It's the furthest thing from a cult," he laughs. "First of all, I don't charge anything for what it is I do. And cults isolate people. What I'm talking about, what I strongly suggest to people, is that they should get closer to the people they're with, and of course cults don't suggest people go to therapy to deal with their issues."

Critics - parents predominantly from America and Canada where most members come from - say people do pay (there's a $10/£6.40 monthly subscription fee and you get special levels of access according to how much more you donate, with $500 buying you the status of "Philosopher King"), and that deFOO proves that FDR does isolate people - the only people members get closer to are each other, and by the time people go into therapy, it's probably too late - they've already decided they were abused and persuade the therapist as such.

Some FDR members have indisputably horrific childhood stories. Some were beaten, others sexually abused. To cut off their parents may well be their only hope for happiness. But if you consider people of Tom's age, who invariably feel their parents don't understand them, and couple this with a youthful thirst for neat philosophical answers to life's problems, then you can see both the attraction and dangers of FDR.

Tom won't talk to me when I track him down, so I try to get a sense of his story from the website. I'm particularly troubled by a live call-in show from April, one month before he left home, in which he aired his very passionate views about animal rights, only to be convinced by Molyneux that he is the one being treated like an animal and abused by his father, and by Barbara because she is his mother and she didn't leave his father - and for even having Tom at all.

Now, let's be clear: Tom does say that he is frightened by his father's mood swings, which sometimes cause him to throw things or shout at the cat. But the conclusions Molyneux jumps to, his manipulation of the conversation, is chilling.

The American parents who talk to me do not want their names printed, and Tom's ex-girlfriend's parents won't talk to me at all. The advice from cult experts is that when a parent attacks or criticises a cult, it may drive their family member further away. I discover this for myself when I see Molyneux in the chatroom telling Tom: "She [Barbara] misses having a victim around and so she is using the media to victimise you ... Totally evil."

Barbara is unfazed, saying that things had already reached rock bottom the moment Tom left home. Her marriage has since broken down and the only good thing that has come out of all this is her relationship with Nick. "We used to talk in terms of, 'I've got post for you' or 'Can I have some money?' Now we show affection and we're really talking," she says.

Molyneux tells me that deFOO is not inevitably for ever, but most members seem to see it as absolute and in one of his podcasts, Molyneux says people who do return to their family risk being seen permanently as unstable.

Some people do manage to leave FDR, however, and I point out that Tom is only 18. Barbara takes a deep breath. "Tom is very strong-willed, much like I am, and when we set our minds to something, we can do it. He is capable of just not coming back."

The only time she doubts this is in her dreams. "Sometimes I dream that Tom is standing in front of me, smiling, and I feel happy and peaceful. But then I wake up."