Luke Harlow, 28, of Ibstock, Leicestershire, admitted grooming and sexually touching Kayleigh Haywood but had denied taking her captive.
Stephen Beadman, 29, who admitted raping and murdering Kayleigh, was found guilty of imprisonment on Monday.
Both men are due to be sentenced at Nottingham Crown Court on Friday.
More on this story and other news in Leicestershire
Kayleigh's body was found in a field, five days after she was dropped off outside Ibstock Community College, on Friday 13 November.
Her family thought she was joining a friend but she was actually meeting Harlow, who had been grooming her through social media and text messages.
Kayleigh, from Measham, was given "substantial amounts of alcohol" and "sexually touched" by Harlow, according to the prosecution, on that Friday evening.
The following day, Beadman arrived at the flat and the jury agreed Kayleigh was kept prisoner by both men between about 21:00 BST and 03:00 the next morning.
Speaking outside court after the final verdict, Kayleigh's parents said they were "heartbroken and devastated".
"Our hearts have been torn apart," they said in a statement.
"We ask ourselves every day, why? Why Kayleigh? Why did this happen to our beautiful daughter?
"Today, we have got justice for our beautiful daughter. But that doesn't change anything for us. We have to live with this heartache forever.
"The public have been and continue to be a great support and comfort to us, and we want to thank each and every one of you."
The court heard how Harlow became Facebook friends with Kayleigh just two weeks before her death.
He had more than 2,200 friends on Facebook, and police believe many of these were girls and young women he was trying to groom.
He admitted two charges of attempting to meet children following sexual grooming, in relation to two other 15-year-old girls he tried and failed to meet.
Janine Smith, Chief Crown Prosecutor at CPS East Midlands, said Kayleigh suffered "a terrifying ordeal" at the hands of Beadman, but was only in that position because she had been groomed by Harlow before being kept against her will.
"Beadman callously raped and took the life of an innocent teenage girl," said Ms Smith.
"Harlow pursued a succession of underage girls in a sinister manner, talking about 'kidnapping' them and making them 'slaves'.
"I would like to offer my heartfelt sympathy to Kayleigh's family and I hope the conviction of these highly dangerous men will in some way help them start to cope with their loss."
The NSPCC said the "shocking" case was a "stark reminder of the dangers that young people face on social media" - and called for more resources to help police tackle the problem.
"Last week, the NSPCC revealed that more than 3,000 sex crimes using the internet were committed against children last year, and Kayleigh's tragic death is an example of the way the online world is increasingly being used by abusers," an NSPCC spokesman said.
"In the coming years, it is vital that police are given the resources to tackle this rapidly developing, 21st century crime, and that we continue to help our children stay safe online."
Paul Burnett, independent chairman of the Leicestershire and Rutland Local Safeguarding Children Board, said a serious case review is under way "to see if any lessons can be learned and action taken".
31 October - Luke Harlow begins grooming Kayleigh Haywood on Facebook, sending her a message reading: "Hey, how's it going?" Further electronic messaging, which she kept from her parents, then takes place on Facebook and WhatsApp.
13 November - Kayleigh's father drops her off outside Ibstock Community College at about 18:00 GMT, after she agrees to spend the weekend at Harlow's home in George Avenue, Ibstock. At Harlow's request, Kayleigh tells her parents she is staying with a friend.
14 November - Kayleigh speaks to her parents on her iPhone in the morning. At about 21:00, Kayleigh's mobile phone ceases to send messages to her friends.
15 November - Between 03:00 and 03:30 a witness sees a young woman being pinned to the ground by a man who is wrongly assumed to be a policeman because of a reflective strip on his coat. A loud frightened scream of "mummy" is heard by another witness as Kayleigh is attacked by Stephen Beadman and then killed on farmland.
At about 9:30 Kayleigh's smashed iPhone is found on a main road near the defendants' homes. A passerby who finds the handset uses the Sim card to dial the last number called and gets through to a friend of Kayleigh, who in turn contacts the schoolgirl's mother. Kayleigh is then reported missing.
16 November - Detectives arrest Harlow and Beadman in connection with Kayleigh's disappearance as officers search for her on parkland and at a property.
17 November - Police appeal for anyone with information or sightings of the schoolgirl to come forward. Later the same day, Kayleigh's parents - Stephanie Haywood and Martin Whitby - make an emotional appeal through police for her to come home.
18 November - Leicestershire Police announce that officers are treating the disappearance of Kayleigh as murder after finding items of clothing they believe belonged to the teenager in Ibstock and Diseworth. Police find Kayleigh's body in undergrowth near a stream at about 22:00.
19 November - Leicestershire Police charge Beadman, then aged 28, with the murder of Kayleigh Haywood. Harlow, 27, is accused of grooming and two counts of sexual activity with a child.
A man who used Facebook to groom a 15-year-old girl who was later raped and murdered has been found guilty of false imprisonment.