By now most of us have learned not to open messages online from people we don't know.But that sometimes goes out the window on social networking sites like Facebook and Twitter, where hackers are often posing as your friends to wreak havoc on your computer.CBS News correspondent Susan Koeppen has that story. (KOH-pen)BONNIE YOUNG JOINED FACEBOOK TO KEEP UP WITH THE BUSY LIVES OF HER DAUGHTERS AND GRANDCHILDREN...(Super: Bonnie Young / Facebook Account Was Hacked)I loved it. I really needed an intervention, a facebook intervention.PRETTY SOON SHE WAS CONNECTING WITH OLD CLASSMATES AND COLLEAGUES... AND HAD 900 FRIENDS.BUT SHE SUFFERED MAJOR HUMILIATION WHEN HER FACEBOOK ACCOUNT WAS HACKED.(01 02 08 - Bonnie Young)well I call it a CTD -- a computer transmitted disease.BONNIE HAD CLICKED ON A LINK SHE THOUGHT WAS FROM A FRIEND WHICH READ, "WOW, LOOK AT THIS VIDEO OF YOURSELF." THAT LINK WAS A VIRUS... WHICH IN TURN SENT THE SAME MESSAGE TO BONNIE'S ENTIRE FRIEND LIST, APPEARING AS IF IT CAME FROM HER.(01 03 07 - Bonnie Young)I got all these emails from people, "what video?" "i can't open it" i opened it - and guess what it was // it was pornography!SOME OF THE MOST COMMON BOGUS LINKS SAY THINGS LIKE, "CLICK HERE FOR MORE FOLLOWERS" -- OR -- "WIN A FREE GIFT CARD."CYBER EXPERTS SAY SOME HACKERS DO THIS TO CAUSE CHAOS... WHILE OTHERS HOPE IT LEADS TO YOUR FINANCIAL INFORMATION.(02 21 14 - SUSAN)what happens if you decide to then click on this link that the hacker has sent you?(Super: John Abell / Wired Magazine)well any one of a number of things. The worst case scenario is that the hacker can destroy your machine or can secretly take it overSO HOW EASY IS IT FOR A HACKER TO REMOTELY ACCESS YOUR ACCOUNT? WE ASKED AN ONLINE SECURITY FIRM TO SHOW US.(nats) "So I click on this link here..."BY CLICKING ON A FAKE LINK... MALICIOUS SOFTWARE IS DOWNLOADED ONTO YOUR COMPUTER WITHOUT YOUR KNOWLEDGE. THIS ALLOWS THE HACKER TO CAPTURE YOUR SERVER CONNECTION.NOW, THE HACKER CAN FOLLOW THE WEB SITES YOU VISIT AND EVEN RECORD YOUR KEY STROKES.(Super: Vincent Weafer / V.P., Symantec Security Response)16 27 28Think of it as being the attacker is literally watching over your shoulder, watching everything you type in that machine, and that's what they have access to. (27 34)FACEBOOK AND TWITTER DID NOT RESPOND TO OUR REQUESTS FOR A STATEMENT.BONNIE YOUNG SAYS BEING HACKED COST HER MONEY AND BUSINESS. SHE PAID 250-DOLLARS TO CLEAN UP HER COMPUTER....AND THINKS TWO OF HER CLIENTS DROPPED HER BECAUSE OF THE INCIDENT.(Bonnie Young)i learned a very big lesson, and it was expensive!EXPERTS SAY THAT A HEALTHY DOSE OF COMMON SENSE IS STILL ONE OF THE BEST FORMS OF PROTECTION.02 23 36(JOHN ABELL / Wired Magazine)Chances are if you get a link from grandma saying join my bellydancing club, dont click on it -- unless grandma happens to be a pro.***Susan Koeppen, CBS News, New York