Three things likely going on here: loss leading, promotion and a hunt for yield.
Let's start with the hunt for yield. Yesterday's auction priced the 3-year at 0.87% and the 5-year at 1.15% [1]. We don't know the term of Zuckerberg'a mortgage. If it was less than 5 years, the bank might make a spread.
If it's a loan with a longer term the lender could have made more by lending to the U.S. Treasury. In that case, the difference may be booked as a promotional expense. "We're the guys Mark Zuckerberg gets his mortgage from" is succinct and memorable.
Finally, the lender may eat a loss on the loan for the opportunity to do more business with Zuckerberg in the future. One sees this with credit facilities in investment banking: JPMorgan and friends give companies cheap loans in hopes of winning their more-lucrative IPO and debt capital markets business.
[1] https://www.treasury.gov/resource-center/data-chart-center/i...