Some thoughts: -Billionaires primary derive their net worth from stock, which is difficult for them to sell. Paper gains, don't represent increases in cash flow. Sources of cash flow would include dividends and planned stock sales. Given the general plunge in dividend yields (4-5% pre-crisis, ~2% today) it's not hard to imagine that even billionaires have less cash at their disposal. Furthermore, given the market plunge and uncertainty the last few years, it would be hard for owners of major public companies to sell large quantities of their shares without signalling to the markets that they believe their companies are overvalued.
-Tax rates and deductions have changed. Shouldn't be a surprise that given the dramatic increase in marginal tax rates and the new limits on deductions that there would be an impact.
-Perhaps there are exogenous factors at play... maybe after a rush of great new non-profit ideas earlier in the decade, billionaires are seeing fewer ideas that excite them enough to donate significantly despite the obstacles mentioned above.
Article just strikes me as wealth bashing without even attempting to get into the more interesting part of the story.