Facebook is extending its experiment charging users to send messages, offering users access to the accounts of VIPs – including Facebook founder Mark Zuckerberg – for as much as $100 (£61) per message.

The offer was spotted by Mashable reporter Chris Taylor who tried to message Zuckerberg, but was told the message would be routed to the less visible "other" folder, unless he paid the $100 fee.

A Facebook spokesperson confirmed to Mashable that the company is testing "some extreme price points to see what works to filter spam".

The cost of sending a message is thought to vary according to the popularity of the VIP. The move follows the introduction last year of paid-for posts, as the company tries to increase revenue from its 1 billion users.

The trial began in December when Facebook introduced a $1 charge to send a message to the Facebook inbox of someone who is not already a friend, though capped at one paid message per recipient per week. Facebook said at the time it was an "economic signal to determine relevance" and help organise users' messages.

"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 explained in a blog post. "For the receiver, this test allows them to hear from people who have an important message to send them."