This should have happened for a long time. Facebook's properties are the places where my friends naturally ask for money from me to help. Even with all the negativity I see here in other comments, this is the natural way for Facebook to grow, not trying to compete with the Federal Reserve.
In an ideal world I'd love to see social media apps be able to loosely hook in to banking apps. So facebook messenger would handle requests and you would tap "send" in facebook messanger which would open your bank app with something like "Send $10 to facebook user ___ and let facebook confirm transaction success" so facebook never has to hold your money or make transactions, but you can make payment requests in messe…
The cool thing with USD payments over lightning network is that capital gains tax is not triggered.