Adoption agencies are reporting huge numbers of calls from "deeply distressed" adoptive parents whose children have been contacted out of the blue.
Social networking sites make tracking down estranged offspring so easy that some birth parents find it impossible to resist their curiousity.
But unsolicited contact flouts adoption guidelines and is throwing youngsters lives into chaos. Many adopted children were abused or neglected by their birth parents before being taken into care.
Now, Britain's leading adoption charities have voiced their concerns that parents flouting guidelines are causing a breakdown in the relationships between some children and their adoptive families.
In some cases children have run away from home without knowing their full history after being lured back to their birth parents by the Facebook messages.
Many of the children were not cared for properly by their parents because they were taking drugs, or were physically or sexually abused.
In many cases, the birth parents dispute the removal, blaming social services.
One message sent to a child given up some years ago for adoption read: "Hello, I'm your birth father. I have been searching for you ever since you were stolen by social services. You look beautiful. I love you so much."
Another read: "Darling son, I am so happy because I have found you here. I have been looking for ages. Please write back because you've been told lies about me."
Many local authorities are now advising adoptive parents not to include photographs in their annual letters, in case these are posted online in an attempt to trace the child.
One adoptive mother said a message to her daughter from the biological mother had had a catastrophic impact on the family.
The adoptive mother, who cannot be identified, said: "Our daughter, who is our prime concern, has gone from no contact from her birth family, at the hands of whom she had a difficult start in life, to suddenly finding they are there at the press of a button."
She said her daughter had just turned 16 and had gone through a whole range of emotions and that it had "completely thrown her".
The natural mother failed to acknowledge why her daughter had been removed from the family at the age of seven.
"She was subjected to abuse and neglect over a long period of time," said her adoptive mother. "But none of that is being acknowledged now."
In another case, a teenage girl was contacted by her biological mother who, in turn, put her in touch with her birth father. The girl was unaware that the man had sexually abused her when she was a young child.
One biological father, whose children were adopted seven years ago, told the Guardian that he used social networking sites to "follow them through life", although he had not sent any messages.
Chris Smith, who believes his children were unfairly adopted, said he wanted to know about their wellbeing. The annual letter does not tell you about their health or interests, he said.
"Because I know where they are, I can just sit and see some of the photos of their school and of events and know they are doing OK," he explained.
Jonathan Pearce, chief executive of Adoption UK, said that unplanned contact through Facebook has become the biggest problem that adopted children are now facing as teenagers.
"Unplanned and unsupported communication, contact and reunions between adoptive and birth families via Facebook and other social networking sites has already had a dramatic effect on adoption," he said.
This will only increase in the future and will mean a radical rethink of how we arrange and support adoptions from care.
The law at present only allows adopted children to apply for their birth certificate and access to court records when they are 18, or 16 in Scotland.
They are given counselling beforehand and if contact between them and their birth parents takes place, it is done in a planned way. Birth parents are not entitled to access records.
Mr Pearce said it is now necessary to prepare children in advance of possible unplanned contact from their birth parents.
"Currently adopted children tend to be told a rose-tinted version of what really happened," he told The Times.
"Something closer to the truth will better protect and prepare children for the destabilising effects of unplanned contact, which often happens at a key stage in their adolescence."
But it is not only parents tracing their children that is of concern. Curious adopted kids, too, are at risk.
The British Association for Adoption and Fostering found that 53 per cent of adopted children have used unofficial means, such as Facebook, to trace their birth parents.
A quarter said that this had been unsettling.
In one case a brother and sister returned to their mother, who they were taken from as toddlers, and stopped all contact with their adoptive parents.
In another case, adoptive parents said their daughter started smoking, drinking and sleeping around after her life became derailed following contact from her birth family.
A Facebook spokesman said: "Protecting the people who use Facebook has always been our top priority."We provide a safe and more trusted online environment by design, offering people industry-leading tools to control what they share and with whom they share it.These controls help protect children and parents involved in adoption from unwanted contact online. "We encourage adoptive and foster parents and children to adjust their privacy settings to control who can contact them on Facebook.
"Unlike many other communication platforms, Facebook also has a robust reporting system in place and our users do and can report people who are contacting them without due consent."
If you need advice about social networking for biological and adoptive families, read our tips from Adoption Search Reunion.