Gavin Scoular, 24, began preying on youngsters as a schoolboy and continued his serial offending for three years.
He identified victims through Facebook, Snapchat and Skype and raped them at his home in Niddrie, on waste ground behind the Royal Infirmary of Edinburgh and in Holyrood Park.
Ex-swimming instructor Scoular, who gave private lessons at the Commonwealth Pool, used social media sites to make contact with his young victims and offer them “affection”.
Scoular was earlier convicted of seven rapes against four girls, as well as a string of other sexual offences, taking the total number of victims to seven. Judge Lord Summers sentenced Scoular to a total of 12 years behind bars at the High Court in Edinburgh yesterday.
He handed him an extended sentence involving a further five-year period of supervision following release from custody. Scoular was placed on the sex offenders’ register indefinitely. The court heard how Scoular’s victims, all of whom were younger than him, ranged in age from 12 to 17.
The offending behaviour in this case began when he almost 15, covering the period from 2010 to 2013.
His offending was brought to a halt when he was arrested in connection with other offences. Scoular was found guilty of the rapes at a trial at the High Court in Livingston last year. Three of the rape victims were under the age of consent when the attacks took place.
Scoular, listed as a prisoner in Dumfries, was also convicted of other charges, including unlawful sexual activity with someone under 16, sexual assault and grooming charges.
Detective Chief Inspector Martin Maclean said of Scoular: “The Public Protection Unit investigation into his devices revealed offending on a scale amongst the worst I have ever come across in such a short space of time.”
Passing sentence, Lord Summers also spoke of “the industrial scale of the offending”.
A spokesman for children’s charity NSPCC Scotland said: “It is clear that children face grave risks in the online world and the NSPCC has called for stricter guidelines.”