Nowadays, it's easier than ever to send money via an app, making cash and debit cards a thing of the past. The best thing about using Facebook to send money is that, unlike other popular services, it's free whether you send or receive money. Other services ̵
Note: You must be friends with the person you want to pay or ask for money from. The service is also available only in the United States, France and the United Kingdom.
Paying or Requesting Money in the Messenger App
Step 1: Start a conversation with a friend you want to pay.
Step 2: Tap the extra sign in the lower left corner.
Step 3: Find and tap Payments on the left
Step 4: Enter the amount you want to pay or request.
Step 5: If you are paying someone, select either Add New Debit Card or to PayPal.
Step 6: Enter your debit card information
Pay or request money from a desktop computer
Step 1: Start a conversation with the friend you want to pay.  Step 2: Locate the dollar sign in the lower right corner of the conversation
Step 3: Enter the amount you either want to pay
Step 4: If If you pay someone you will be asked to enter your debit card information.
Step 5: Click Pay at the bottom. right corner
How to Receive a Payment
You will receive a notification after receiving a payment. If you are doing this for the first time, you will need to provide your debit card information. Once that's done, the money goes directly to the checking account associated with your debit card.
Setting a Pin
Facebook may already be encrypting your payments and user information to protect your transaction details – a good idea. Fortunately, you can now set a PIN on Facebook that requires you to enter a four-digit code each time you make a payment. Currently it's only available in the mobile app, but it's easy to set up.
Step 1: Scroll down the menu area of the Facebook app to Settings and Privacy and tap Settings .
Step 2: Select Payments
Step 3: Type in Security PIN PIN 
Step 4 : Enter a four-digit PIN that you will remember.
Step 5: Enter your PIN again when prompted.
Currently, Facebook only allows payment with debit cards. It's also important to know that it may take up to five days for your bank to approve the transaction, but it's definitely a great way to share a bill or repay your roommate for food.