"The 21st century fraudster's job has been made much easier because of our technology, particularly the Internet," he said. "A fraudster who is half a world away can now get a pretty good profile of any one of us, just by reading postings that we put on bulletin boards, by emails that are out there and just by seeing other activity that we're involved in, in our ordinary lives." It's a high-tech advantage that they have over criminals from just one generation ago.
"They no longer necessarily have to have a particular fraud in mind," Leff said. "They almost have an entire catalog full of frauds ... and based on what someone's Internet activity is, it enables them to decide which victim is best suited to be perpetrated by a particular type of fraud."
Leff said that as soon as someone makes an entry on to a bulletin board, someone somewhere is reading it, in places as remote as Nigeria, the Philippines or the former Soviet bloc.
"They're trying to figure out, 'What age is this person?' " he said. " 'Are they lonely? Are they looking for business opportunities?' And that's the type of fraud that that particular devious criminal is going to look to perpetrate against you."
So what can you do about it? The FBI's best advice is simple.
"The same way you probably wouldn't have someone knock on your door and say they want to be a part of your life and just let them come right on in, basically don't introduce someone to your finances, to your checkbook," he said.
"Don't introduce them to your credit cards unless you've been introduced to their family, their co-workers, their friends, at least to some extent. If it's a distance relationship, then treat it like a distance relationship and keep them at a distance from your finances. Otherwise you could wind up in a bad way."
—By Liza Hughes, Special to CNBC.com. Follow Liza on Twitter at