People seem to get the weird idea that borrowing against their stock holdings is some special thing rich people get to do with products that the rest of us don't have access to. It's not. Margin loans are widely available to the tune of ff+1%ish or lower, and if your brokerage's publicly offered rates are probably a ripoff, they're almost certainly negotiable. The bar for access to "institutional" rates is basically 100k, the regulatory requirement for portfolio margin.

Yes, there are specialized products catered to billionaires. But those aren't getting them better rates than someone with a $200k portfolio (Zuck is not conventionally a less risky borrower than the Options Clearing Corporation!). They exist to work around the fact that some borrowers can't just casually liquidate their stock on the open market, let alone at face value. By all accounts these products are more expensive than retail.

Mostly this is an expensive (but maybe still less expensive than taxes, depending on the rate environment—it's more of a no-brainer in ZIRPland) way to diversify out of a single-stock portfolio without selling by adding leverage. At Zuck's age, it's still very unlikely to make sense to borrow instead of sell to spend. He's been known to pay real taxes in the past, they just look small relative to his imputed wealth growth because rich people don't spend a lot relative to their wealth growth because they, quite by definition, have a lot of wealth.