E-mail accounts and websites were also targeted by young hackers.
One in four young Britons attempts to access the Facebook accounts of their friends, a survey claims.
The most common route of access was by working out - or "cracking" - each other's passwords.
The poll of 1,150 under-19s found that nearly half of those who accessed other accounts did so from either their own computer or one at school.
The main reason given for doing it was for fun, and a further 21% admitted they hoped to cause disruption.
The young people questioned took part in the online survey anonymously.
78% of them said that they knew that hacking was wrong and 82% said they found it difficult to do in practice.
Reuven Harrison, co-founder of Tufin Technologies which commissioned the survey, told the BBC that young people need better education in order to understand when hacking is unacceptable.
"Playing around with computers and trying to understand the system can be leveraged for good and bad purposes," he said.
"There's a fine line at which point it becomes something bad. Children don't always understand where that line is."
20% of those who admitted to hacking in the survey believed they could make money from the activity and 5% described hacking as a career option.
"Hacking into personal online accounts can be child's play if users do not protect their own passwords," said Deputy Chief Constable Stuart Hyde, president of the Society for the Policing of Cyberspace.
"Hacking is illegal and we need to ensure everyone understands that."