it’s not just UAL and Delta
Delta made about $7.4 billion from American Express (AXP) last year, "driven by high single-digit growth in co-brand spend and over 1 million new card acquisitions," President Glen Hauenstein said on the company's fourth-quarter earnings call.

American signed an exclusive deal with Citi (C) on co-branded credit cards in early December, sending its shares up 17% that day. The carrier, which said it received $5.6 billion from its co-branded credit card and other parters in the 12 months ended Sept. 30, expects that number to keep growing, eventually approaching $10 billion.