I had no idea how rapidly this was growing. From No Such Thing as a Free Gift, by Linsey McGoey, loc 282:
Nearly half of the 85,000 private foundations in the United States alone were created in the past fifteen years. About 5,000 more philanthropic foundations are set up each year.
There are questions that can be raise empirically about whether this represents an overall growth in charitable giving. But the evidence of elite philanthropy seems unsalable. As she observes on loc 302:
The surge in global philanthropy is rooted in growing wealth concentration, something that has enriched the ability to give away eye-popping sums.