Facebook is introducing sweeping changes to its messaging system with a new set of filters, including one which charges people $1 to message anyone on Facebook they don't already know.

Calling it a "small experiment to test the usefulness of economic signals," the social network says the changes are primarily designed to fight spam.

"This test will give a small number of people the option to pay to have a message routed to the Inbox rather than the Other folder of a recipient that they are not connected with," said Facebook on its Web site.

"Several commentators and researchers have noted that imposing a financial cost on the sender may be the most effective way to discourage unwanted messages and facilitate delivery of messages that are relevant and useful.

"This test is designed to address situations where neither social nor algorithmic signals are sufficient."

As things stand, Facebook forwards any message sent to one of its users to their Inbox if they know the person, otherwise it goes into the abyss of the "Other" folder. Facebook believes that if the message from the stranger is important enough, then they'd be willing to pay a dollar to make sure it arrives with the intended recipient.

"For example, if you want to send a message to someone you heard speak at an event but are not friends with, or if you want to message someone about a job opportunity, you can use this feature to reach their Inbox," added Facebook. "For the receiver, this test allows them to hear from people who have an important message to send them."

At present the new filter is just a test being rolled out to Facebook's U.S. users. The social network is limiting the number of messages a person can receive this way to one a week.