Facebook users are being warned about a new application on the social networking site that contains adware.

‘Secret Crush’ contains a download of the Zango adware program which automatically sends itself to five friends.

To find out whom their flame is, users must download a widget, he said. Then, they must invite five other friends to install the same widget as part of a “manipulation strategy,” Lovet said.

“It’s what we call a social worm,” he said. “Unlike traditional worms, it doesn’t spread by using network APIs (application programming interface) or computer code. It spreads by manipulating human users.”

The Secret Crush application, created by an Australia and U.S.-based firm called Mobile Messenger that also runs the My Luv Crush site, shows up as a request in a user Facebook application from a friend. The message implies that one of your friends has a secret crush on you.

Once you add the application to your profile, the application’s terms and services says the company will charge $1.25 a day to send SMS horoscope messages to a Facebook user’s cellphone if it is provided. Users complained more than a month ago about the service, but did not mention the installation of adware.

“Businesses need to adjust their security and usage policies to address the realities of the Web 2.0 world,” said Paul Henry, vice president of technology evangelism at security gateway maker Secure Computing.