Using a pseudonym on Facebook, a security guard befriended four girls aged between 11 and 13, including a pair of twins he was two-timing, and sexually exploited them.

Benjamin Sim Wei Liang, 30, who admitted to a slew of offences - including statutory rape - against the girls between November 2012 and January 2013, was on Wednesday sentenced to 201/2 years in jail and 24 strokes of the cane.

He sexually violated the girls in public toilets, on staircase landings and even in the back seat of a taxi. He also asked them to send him pictures of themselves naked.

And in the case of two girls, he took them to his parents' condominium in Woodleigh Close, off Upper Serangoon Road, and had sex with them in his bedroom.

The High Court heard that Sim first got to know the older twin in November 2012 through Facebook, using the name "Peter Tan". They exchanged phone numbers, and she agreed to be his girlfriend.

In the first few weeks of the relationship, Sim asked her to send him pictures of herself naked, and she did. After she told her twin about Peter Tan, the younger girl added him as a Facebook friend. On the same day, he asked her to be his girlfriend. She said yes.

Over the next two months, Sim met the sisters separately, and bought them each a mobile phone.

He groped the older twin in a taxi and outside a school. He went further with the younger twin - he groped her on a bus, abused her in several public places, and made her perform oral sex on him. Twice, he took her home and asked for sex.

All this time, Sim continued to befriend other girls on Facebook. He asked an 11-year-old to show him her breasts; she sent him two pictures - one of herself and one of her cousin's - and a video.

He also befriended a 13-year-old girl and had sex with her.

On Dec 15, 2012, the twins' mother found the cellphone Sim had given the younger girl and took her to make a police report. The girl initially told the police she was raped but later said the sex was consensual.

Deputy Public Prosecutor Zhong Zewei asked for a heavy sentence, given the brazen acts against the young victims. Sim's lawyer Edmond Pereira said the Institute of Technical Education dropout has below-average intelligence, and believed it was not wrong to have sex with the girls if they consented.