Member since … well, never.

Facebook, which today unveiled its long-awaited peer-to-peer payments, notably has left out American Express — no doubt much to Visa and MasterCard’s delight.

Facebook said today that its Messenger service will start allowing payments between Facebook “friends.” The P2P service is being rolled out slowly across the US.

To use the service, Facebook members will need a debit card or bank account. Well, a Visa or MasterCard debit card, that is. No American Express cards, or Discover cards for that matter, will be accepted.

Here’s what Facebook said:

The first time you send or receive money in Messenger, you’ll need to add a Visa or MasterCard debit card issued by a US bank to your account. Once you add a debit card, you can create a PIN to provide additional security the next time you send money. On iOS devices you can also enable Touch ID.

Running on the debit network rather than credit cards gives Facebook a lower wholesale cost to begin with, and the company likely received highly advantageous deals from the networks. The whole premise around that lower wholesale cost is volume. Lots of volume. Facebook has 1.2 billion users, and presumably a chunk of them will use the “Send Money” feature, as it is called — particularly since the service is free.

That Amex is shut out of that business has got to sour the payments company. Amex pushes the Facebook brand through its dedicated “Amex Offers on Facebook” service. And Amex started pushing alerts on Facebook to its cardholders in 2013.

Further, this lost deal is just another dark spot for American Express so far this year. It has lost JetBlue and Costco has clients, and it is being forced to lay off 4,000. But perhaps Amex will get the last laugh? At the end of last year, it launched a “technology hub” in Palo Alto, Calif. By car, a 12-minute drive from Facebook’s Menlo Park headquarters.