Jason Downie, 32, befriended youngsters on the social media site before sending them messages asking to meet up.
Aberdeen Sheriff Court heard how he sexually assaulted a 13-year-old girl in Fraserburgh in September 2012.
Sentencing him, Sheriff Annella Cowan described Downie as a "dangerous and manipulative" man who would remain on the sex offenders register for life.
Downie, who admitted the assault, had started chatting with the young girl on Facebook and X-box Live Chat.
The court heard he then encouraged the girl to meet him in a car in Fraserburgh.
Fiscal depute Kelly Mitchell said: "The complainer asked if he would get her a sweet from the nearby shop. He said that he would do it in return for a kiss."
After seizing the youngster's head and kissing her he jumped into the back seat of the vehicle and sexually assaulted the schoolgirl.
Miss Mitchell said: "She screamed 'stop' and 'get off me'. Neighbours stated that they heard a girl screaming."
The court heard that the next day the girl went to the police and reported the attack.
Downie also pleaded guilty  to travelling "with the intention of having unlawful sex" with another young girl he befriended on Facebook earlier this year.
The court heard he sent her inappropriate messages in June 2015, asking for pictures of herself in her underwear.
At one point, he suggested they go for a drive in his car and they could kiss - but the girl turned up to the meeting near Banff in the company of friends.
The court heard Downie later sent her abusive messages, stating that she had wasted his "time and fuel".
The  girl's mother found the messages on her daughter's Facebook app and reported Downie to the police.
Downie, whose address on court papers was given as HMP Grampian, was sentenced to 38 months in jail, with an extended sentence of three years.
A man who used Facebook to find young girls to have unlawful sex with has been jailed for more than three years.