i don't think it is impossible to keep it away from society. for someone that rich it is possible to keep it as a secret. besides, as you see in the OP their names and how much they gave away is used for a purpose and that could never happen without permission. what do you think?
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For one, to answer your last question, I don't know the legal ramifications of using their names and images in an advertisement. I'm doubtful that either Gates or Buffett agreed to be in these ads. The information of how much they gave is public information.
For two, let's consider the options these guys have.
Gates:
Gates built his fortune on Microsoft stock, by founding the company and then holding onto a large percentage of it as it skyrocketed in value. When an insider owns that much of a company, it must be disclosed in their SEC filings. In addition, when an insider buys or sells stock, I believe that also must be disclosed.
At one point, he formed the Gates Foundation, and he significantly decreased his activity at Microsoft in order to focus on his foundation. He runs the foundation, so there's no way he can hide what's going on. Much of his wealth has gone to it. If he sold Microsoft stock, it must be publicly disclosed.
Buffett:
Buffett has always been an investor, and over the past several decades (since the 60s), he's been Chairman and CEO of Berkshire Hathaway. He owns a massive percentage of the public company, and again, this legally must be disclosed. He long ago vowed not to sell Berkshire stock. And, if he were to sell it, he would encounter capital gains taxes (thereby decreasing the total amount he can give to charity), and it would be disclosed to the public and his shareholders, who would demand to know why the primary insider is selling billions of his company stock (like, well, Enron executives did). So, he can't just hide where $60 billion in company stock goes. Years ago, he pledged something like $30 billion to the Gates Foundation through a series of Berkshire stock gifts. Based on the amount that these stock gifts are worth, they almost certainly have to be included in SEC filings.
In order to abide by laws and to distribute the wealth most efficiently, there's basically no way these guys can hide what's going on. We're talking about many billions of dollars here. Their combined net worth is greater than most small countrys' GDP values.