All that comes to mind is that he is now surpassing the net worth of Bill Gates, to a large extent because when he left Microsoft he said he would keep his Microsoft stock positions.
I think Gates has given away ~$50B - which would seem to be enough to tip the scales? I'd rather compare the billionaires by what they redistribute vs. the size of their hoard.
Bill Gates controls the Gates Foundation; if you're trying to count his wealth, you should include its endowment, which it currently reports as $75 billion. (And it also reports that it has made $77 billion of grants since inception, so if you want to include spent money too, he's outclassing Ballmer without even considering his personal holdings. It's more consistent to just count held money.)