SAN FRANCISCO (Reuters) - Facebook Inc unveiled a test on Thursday that charges users to send certain types of messages through the social network, the latest example of the company looking for new sources of revenue and profit. Until now, Facebook's messaging system sends the most-relevant messages, including those from users' Facebook friends, into an Inbox and siphons off less-relevant messages, such as potential spam, in an "Other" folder. "Today we're starting a small experiment to test the usefulness of economic signals to determine relevance," Facebook said. ...

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