So you're saying you understand the value proposition and that it's been successful elsewhere, but you don't understand why Facebook would want to do it, or why it might make sense for the company who already has messaging apps on billions of phones?
Doesn’t Transferwise already fill this niche? These days you can send several hundred dollars to and from the US for around $2-4 in fees, whereas the fees used to be $40-60 from traditional banks or Western Union.