Facebook has begun a pilot scheme which allows users to pay $1 to send messages to people they are not friends with.
The social networking site has introduced the fee in as trial which it insists is only for a "small number of people" and is initially being tested just in the US.
The Facebook messaging system normally deflects unwanted or unsolicited correspondence into an 'other' folder that users can ignore, the change, however, could mean that messages will go into the recipient's inbox instead.
Users could receive one paid-for message per week, and no more than three each month.
The trial has sparked fears among some users that it will be exploited by advertisers happy to pay such a low charge to send the messages, and that users would be bombarded with spam advertising.
Facebook uses an algorithm calculation based on social cues to filter messages in terms of relevance, allowing some messages through, for instance those sent from a 'friend of a friend'.
In a statement Facebook said: "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," the site said in a statement.
"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."
In a blog post the company also said: "This test is designed to address situations where neither social nor algorithmic signals are sufficient," Facebook said in a blog post.
"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."Reuse content