That's not true. Americans (Facebook's intended audience for this Messenger money-sending feature) can spend bitcoins directly at Microsoft, Dell, Expedia, Overstock, Newegg, Tiger Direct, DISH Network, etc. They can use and spend bitcoins theoretically received from FB friends without having a credit card, without even having a bank account.
100,000 (legitimate!) merchants worldwide accept Bitcoin - http://www.ibtimes.co.uk/bitcoin-now-accepted-by-100000-merc... It is time to kill the myth of "only the dark web accepts Bitcoin".