Scammers are phishing social networks because they have a better chance of tricking their victims, said Rik Ferguson, a security researcher with Trend Micro who phishing campaign. They "tend to be more successful, because they take advantage of the inherent trust that these systems are based on," he said.

Once criminals have access to these accounts they can make money by sending out spam messages via Twitter or Facebook, or they can re-use the username and password combinations to try to log into other services such as Web-based e-mail, Ferguson said.

On Thursday, security vendor AppRiver reported a new round of Facebook phishing attacks. These messages have the subject line "Hello" and read "Check areps(.)at." This scam, which tries to steal Facebook usernames and login credentials, also promotes the bests(.)at domain. (These domains are also unsafe to visit)

Another reason why Twitter spam is so effective is because Twitter users rarely know what Web sites they're going to visit. Because messages can't be more than 140 characters long, senders often use services like TinyURL or UR.LC to shorten their links, hiding the ultimate destination from Web surfers until they arrive at the site.

Victims are often phished without realizing it. Tim Pratt, a freelance writer based in San Francisco, didn't realize he'd been hacked until his Twitter account sent out one of the phishing messages and friends started contacting him.