LOS ANGELES - Facebook announced Thursday it was testing a fee-based messaging service that guarantees messages from strangers will arrive in the inboxes of intended recipients.

The test, the latest move in Facebook's campaign to generate more revenue, charges a dollar to ensure a message is routed to someone's regular inbox even when the person is not in the sender's circle of friends.

Currently, messages of dubious interest to recipients normally go into an "other" folder, which is often ignored.

Charging for such messages could help discourage spammers and might be useful in certain situations, Facebook said.

"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," Facebook said in an online post. "For the receiver, this test allows them to hear from people who have an important message to send them."

The test was limited to a small number of users in the United States, said the California-based social network.

Facebook has been under growing pressure to tap new sources of revenue, with its shares trading at $27.36 at the close of trading on the Nasdaq exchange Thursday, way below the initial public offering price of $38 in May. ? ?

In response, the company has introduced a series of new services and trials in recent months. In October, Facebook unveiled a feature in which, for $7, users could make personal posts more visible in their contacts' newsfeeds.