A Perth man has been sentenced to more than five years' jail for repeatedly raping a child he met on Facebook, and forcing young girls to send him naked pictures of themselves that he then threatened to post on social media.

Key points: Rhian Lee Ryan posted more than 330,000 messages to Facebook from age 14-19

Rhian Lee Ryan posted more than 330,000 messages to Facebook from age 14-19 His flirty messages to girls escalated to threats, blackmail and sexual assault

His flirty messages to girls escalated to threats, blackmail and sexual assault The Judge said Ryan's messages showed a "disdainful" attitude to females

District Court Judge Stephen Scott said the "extraordinarily disgusting" messages Rhian Lee Ryan sent to the girls, and his "complete disdain for females", further added to the trauma and profound distress his crimes had caused them.

Judge Scott said the social media giant needed to do more to protect its users from being exposed to such extreme bullying, and the message the sentence delivered to the community was "those who steal the innocence of young girls need to be properly punished".

Ryan, who is now 21, had been found guilty of sexually penetrating a girl who was 14 years old in January 2015, when he was aged 17.

'Flirty' messages escalated to rape

He forced her to send naked pictures of herself, unlawfully detained her and assaulted her causing bodily harm.

He had sent her a friend request on Facebook and after three weeks of conversation became "flirty" and asked her for explicit photos.

When she refused, he called her a "slut" and a "tramp" and threatened to beat her up in public.

Judge Scott said she felt she had no other option than to send the nude photos on Snapchat, which he then took screen shots of and saved copies.

He then told her if she did not meet with him he would send the photos to her family and post them on public forums.

Ryan was sentenced to a total of five years and four months jail. ( Supplied: Facebook )

She then went to his house, where the court was told he repeatedly raped her during that day and overnight.

When she tried to leave he violently assaulted her. He then forced her to stay the night, and she only managed to escape the next morning when he was sleeping.

Ryan was also sentenced for threatening to post on social media nude photos a 12-year-old girl had sent him, after he friended her on Facebook in January 2017.

He was 19 years old at the time.

In August 2018 he sexually penetrated two 13-year-old girls at the home in which he was living after meeting them in a park and friending them on Facebook.

An 'insidious by-product' of Facebook

Judge Scott told the court Ryan used social media extensively to engage sexually with young girls and women, posting 330,000 messages on social media during the period from when he was 14 to 19 years of age.

He said Ryan was evicted from his parents' home at age 13. When he was 14 he sent nude photos of himself to an older girl and she then posted the pictures on social media, leaving him feeling "betrayed".

Ryan later joined a chat group which shared naked images of young girls online.

Judge Scott said his messages showed he had a "disparaging and disdainful" attitude to females, who he only saw as sexual objects.

He said the victims' impact statements "make absolutely clear the offending profoundly affected each of them and will continue to do so".

"Each of the victims has been affected in a most distressing way," he said.

Judge Scott questioned when social media sites would act to protect people from threats. ( ABC News: Tim Leslie )

"They are inevitably going to suffer for a considerable period of time. It's heart-wrenching to read the impact on girls at a tender age."

He did not accept Ryan was genuinely remorseful or had genuine insight into the impact on his victims, adding the only mitigating factor was his age at the time of offending.

Judge Scott was also scathing of social media.

"It's an insidious by-product of Facebook that people can say and do what they like with no consequence," he said.

He questioned when Facebook and other social media sites would act to protect people from these sorts of threats and bullying, which he said had already led to suicides.

Ryan was sentenced to a total of five years and four months' jail, which he will serve on top of a 20-month sentence he is currently serving for sexual offences against a 15-year-old girl when he was 18.