The charge is part of a test of a system designed by Facebook to both raise some revenue and reduce spam.

It was first introduced in December, with a charge of $1 for the sender of a message to reach the inbox of a Facebook member who is not their friend. Facebook said that it wanted to determine whether adding a "financial signal" improves its formula for delivering "relevant and useful" messages to members' inboxes.

Now, in order to reach VIPs with large numbers of followers who are not friends, including Mr Zuckberg, Facebook members involved in the test, which is currently only running in the United States, face a charge of $100.