His girlfriend caught him. Not by checking his phone messages, pockets or credit card bills. Too old school.

Instead, she kept tabs on Michael's whereabouts by sneaking a peek at his Facebook profile -- the latest phenomenon in social networking websites.

Within hours after the 24-year-old drove home another girl from a nightclub, his girlfriend knew about it.

"I once drove this girl home after a night at the club and the next day -- I had no idea she did it -- she posted on my Facebook site: 'I had a great time, thank you for driving me home,'" Michael, a graphic designer, told CTV.ca.

"Before I even knew about it, a couple of hours later, my girlfriend called me and asked 'What the hell's that?'" recounted Michael, who requested that his name be changed.

With more than 19-million registered users, Facebook is the sixth-most trafficked site in the U.S., and has become a growing indulgence among young professionals who use it to keep up with friends, partners and potentials.

Say goodbye to googling. Instead, many are now "facebooking" everybody from old high school enemies to new love interests -- quickly obtaining juicy personal details and personal pictures.

Is there a cute girl from the office you could never find common ground with? Facebook her name, find out her favourite movie from her profile and then, the next time you bump into her in the cafeteria, ask if she's ever seen "The Notebook" and tell her about the part that brought you to tears.

How it works

Sound creepy? Here's how that works and how Michael got busted:

Those who choose to register on Facebook essentially create their own personalized website, complete with a list of their friends, photo albums, and a message board they can choose to make public.

There are also networks you can join that extend your friend base -- anything from the city you live in, the high school you went to or the musical genre that you like. In a network like 'Toronto', you could essentially access and post on any profile that hasn't been limited to "friends only."

To gain access to a friend's profile that is not open to everyone you must first make a "friend request."

But once a user grants another member access to their profile, they have no control over what that person can post.

So, while Michael approved a 'friend request' from the woman he met at the club, he wasn't able to stop her from posting the damaging message on his site. Before he could remove it, his girlfriend had already seen it.

At the same time, had he utilized an option to block his girlfriend from his message board, she would have wondered what he was hiding.

And even if Michael's girlfriend hadn't checked his message board, she may have been alerted to the note on her "news feed."

News feeds allow Facebook users to keep track of their friends without actively seeking out their profiles. For example, when there is a new message or picture posting -- your other friends will receive a note on their news feed. 

Perhaps Michael deserved it in his case, but Facebook can ultimately have more serious implications.

"Let's say I've gone to great pains to make sure that my home address is not on my Facebook account but I'm throwing a party so I tell my friends 'Come over to my place Saturday night,'" technology expert Kris Abel told CTV.ca.

"So all my friends start to organize with each other ... and someone online says 'Hey, I'll be helpful, I'll go to Kris Abel's profile and I'll post his address in the comments section.'

"Of course, 24 hours go by and I check my Facebook account and I realize not only is my address posted in the comment section of my profile but everybody that's going to the party has copied that and posted it in their profiles."

'RSS feeds' on social networking sites are another trend that could have far-reaching consequences on privacy. These feeds function as an ongoing update -- which can be posted from your Blackberry -- about exactly what you are doing: including alerts that you've woken up, eating breakfast or waiting at the airport for a flight.

"It's you reporting on the minutiae of your life," Abel said.

U.S. presidential candidate John Edwards, for example, uses an RSS feed to notify the public about his comings and goings during his campaign stops.

Abel warns that the overabundance of detailed information could be dangerous -- especially in the hands of stalkers or nosy co-workers.

Pictures of drunken escapades or long-weekend revelries can also come back to haunt you if an employer decides to Facebook you before offering you a job.

"It allows people to spy on you and jump to conclusions," says Abel. "Since Facebook tends to be very social, it tends to paint a very socialized version of yourself."

Face It

In just three years, Facebook has managed to morph from a site designed as a way for college students to stay connected into the ultimate spy machine.

It's been used by schools to monitor students accused of cyber-bullying their teachers. There are even conspiracy theories that U.S. intelligence agencies use the information for data-mining purposes.

But its benefits as a social network are debatable.

For keeping in touch: good.

For couples in rocky relationships: not-so-good.

Certified psychotherapist and relationship expert Rebecca Rosenblat, a.k.a. Dr. Date, told CTV.ca that social networking sites like Facebook can prove fatal to trust in relationships.

"When someone's in a troubled relationship they're more predisposed to seeing what's out there," said Rosenblat. "That might be the time when there's the temptation to look someone up.

"Previously, it was a matter of people working a bit harder at keeping their relationship but it's significantly easier now to see what's out there from the comfort of your relationship without actually leaving the relationship to see what your options are."

For couples who are lacking the incentive to work at the relationship they're in, this could prove deadly.

"Most therapists say that anything that takes away emotional and physical energy from your primary relationship is detrimental to your relationship," she said.

Her advice is for people to put themselves in their partner's position and to ask if they'd be upset in a similar situation. Despite being a therapist, Rosenblat says she's still puzzled as to why people in relationships cheat using Facebook.

"It's similar to saying 'Okay, I'm going to cheat but I'm going to hire a private investigator to follow me everywhere,'" she says. "If you give the Facebook (contact), it's like you giving a home phone number and the person calling and your girlfriend picking it up."

His dating drama aside, Michael says Facebook could bolster an already-strong relationship.

"In a way it kind of makes things more open, so especially if you're in a relationship and if there's honesty in it than you have nothing to worry about."