The Facebook Messages test, limited to the United States, lets a sender pay a dollar to make sure an electronic missive is routed to someone's "inbox" even when the person isn't in their circle of friends.

Facebook messaging system was billed as being designed to deflect seemingly unwanted correspondence into an "other" folder that can be ignored.

Facebook said that it wanted to determine whether adding a "financial signal" improves its formula for delivering "relevant and useful" messages to members' inboxes.

Facebook already uses social cues, such as connections between friends, and algorithms that identify spam messages.

"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."

The Menlo Park, California-based social network in 2011 introduced "other" folders as repositories for messages of dubious interest to recipients.

The test was introduced along with updates that included "basic" or "strict" filtering settings for inboxes.

The strict setting limits inboxes to little more than messages from friends at the social network, while the basic setting opens the door to friends of friends.