Venmo, a mobile payments app owned by eBay’s PayPal unit, is perhaps the most direct competitor to Facebook’s new offering. Popular with young users, it is not just a payment system, but a social network that allows users to post public or private messages about what the money is for.

Square, the e-commerce start-up, offers a similar app that allows payments to individuals by email. And Snapchat, the start-up known for its disappearing messages, also allows users to send cash to one another through a partnership with Square.

With its service, Facebook wanted to simplify the process as much as possible, according to Steve Davis, the product manager in charge of the project.

“We know that conversations about money are happening all the time,” he said in an interview. “But most conversations begin in one place and end in another place.”

Facebook wanted to keep the payment and the conversation in one message thread that would also serve as a record. So right next to the thumbs-up button on the Messenger screen will be a dollar-sign icon to send money. If a debit card number is already saved in the app, you can send money to the other person in the conversation by clicking the dollar sign and entering an amount. The whole conversation will be saved for later reference.

To reduce the risk of unauthorized transactions, Facebook said, users must enter a PIN or use Apple’s fingerprint identification system before they can send the money.

By using debit cards to handle the transfer, the money can move fairly quickly between the two bank accounts while allowing Facebook to offer the service free to users. Unlike PayPal or Venmo, “you don’t have to remember to withdraw the funds later,” Mr. Davis said.