Three persons, including the youth and two of his woman friends, have been booked in this connection, they said.
The man, who is a close relative of the 16-year-old victim's father, allegedly created the fake account on Facebook last year.
He then purportedly sent obscene pictures and messages from the fake account to a female friend of his, who is also an acquaintance of the victim, said Senior Police Inspector Gajanan Kabdule of Naupada police station, based on a complaint filed by the victim's mother.
He also called her to a place in Thane's Naupada locality where he allegedly molested and beat her up, police said.
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The man later got the obscene messages deleted from his female friend's account.
He then asked the victim to send him all screenshot messages he sent earlier, on his WhatsApp to resolve the matter.
However, after the victim sent him the messages, the man, on May 26 this year, posted the same on another woman friend's Facebook page, as per the complaint.
Police registered offences in the wee hours today against three persons - the man and his two woman friends - under sections 354 (assault or criminal force to woman with intent to outrage her modesty), 292, 500 (defamation), 504 (intentional insult with intent to provoke breach of the peace) and 506 (criminal intimidation) of the IPC and also relevant sections of the IT Act and Protection of Children from Sexual Offences (POCSO) Act, police said.