A 20-year-old Brooklyn, N.Y., man has been using the popular college-community Web site Facebook.com to con and extort coeds around the country, forcing them to turn over nude pictures of themselves and give him online companionship, officials said.

"Students think that because they're on a secure Web site like Facebook that they're safe from online predators," Manhattan District Attorney Robert Morgenthau said. "They're not."

Elvin Chaung is proof of that, the DA said — an alleged cyber-sicko who got over 50 women to send him nude photos and videos by posing online as one of their girlfriends. Investigators believe he tried the scam on hundreds of college students around the country.

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"He was using identity theft and extortion to get naked pictures of 18- and 19-year-old women," said prosecutor Aaron Karczmer.

Most of Chaung's victims were his schoolmates at Hunter College in Manhattan, but he also worked his manipulative magic on naive teens at over two dozen other colleges and universities, including Columbia University, Karczmer said.

Sometimes the pictures weren't enough. On one occasion, he instant messaged a victim who realized she'd been conned into sending him racy pictures and threatened to humiliate her unless she continued "talking" to him, Karczmer said.

"As of now, I haven't done anything with them. Don't make me change my mind. I just want to chat ... I want to get to know u more. Ur hotttt," Chaung — or "OneSweeetgurlie" — wrote in one exchange, court records show.

If "I were to lose contact wit u I mite want to send the pics all over the internet. As long as we can chat and be friends the pics are safe."

Chaung, who lives with his parents in Ditmas Park, Brooklyn, declined to comment. He's free on bail after being arrested and charged with identity theft, computer trespass and tampering, coercion and grand larceny.

In a recent hearing before Manhattan Criminal Court Judge Abraham Clott, Karczmer said Chaung has been working his scams since this past fall, using his Facebook account as a key to opening doors to coeds around the country.

Facebook is a popular online network that is aimed primarily at college students. It allows people with a college e-mail address to start their own Web pages, post pictures and talk online with friends, and allows them special access to other schoolmates' pages.

Chaung allegedly used his Facebook account to "case" female students, watching their pages to find out who their friends were and whatever he could about their backgrounds.

First, he'd claim to be an old high-school pal or a Facebook friend, using a new instant-messaging account because of "computer problems."

"I been IMin, textin, emaling callin everyone I know — EVERYONE I could think up of," "she" explained, saying "she" would've called but her cell phone was dead, records show.

Chaung would then say he was in big trouble with his photo or art professor and desperately needed pictures e-mailed to him, the records show. He said if he didn't get pictures for his "project," he would flunk the class and lose a $2,000 scholarship.

After getting his victim to send a regular picture, he would then push for a bikini or lingerie shot, and then a nude shot, prosecutors said.

If the victim would balk at sending the nude photo, he'd threaten to plaster the bikini or lingerie shots around their schools unless they complied, records show.

Chaung told investigators the threats worked "approximately 10 percent of the time," court papers say.

In at least one instance, he carried out a threat, filings say.

He doctored a lingerie shot of a woman to make it look like she was naked and posted the pic on her friend's page, court papers say.

Anyone with information on the case is asked to call the Manhattan DA's identity-theft hot line at (212) 335-9600.

Chaung faces 2½ to seven years in prison if convicted.