Facebook has said Messenger extends payments from a one-to-one to now include group payments. It cites and example of how a group payment might be required, such as splitting a restaurant bill, or for a party, or group gift.
Will the old 'I've forgotten my wallet' trick become 'Oh sorry, I'm not on Facebook'? Will people with Facebook accounts not socialise with non-Facebook people for fear of being stiffed with the bill? Will the first date question 'Are you on Facebook?' take on more meaning now?
(Apologies for the frivolity - I haven't had my morning coffee yet)
It sounds like a good idea for FB. No fees .... yet.