funkyD, my remarks are intended to speak of motive, rather than effect.
"Philanthropist" has the connotation of "loves people." In that respect, a billionaire who develops his or her wealth by opportunistically taking advantage of others (yes, that might well be a definition of capitalism), can hardly be categorized as being a philanthropist, notwithstanding whatever percentage of their ill-got gain turns back into charity. It's motive at issue here, not quantity.
Consider what S.I. Hayakawa says about these concepts:
Charitable, humanitarian, and philanthropic all suggest a sense of obligation to aspects of life that are, or are regarded as, worthy of generous understanding and practical help. A charitable person is disposed to show a kindly and merciful attitude toward people in distress and to help them when and where possible. A humanitarian will generalize his interest in mankind along philosophic and often vaguely sentimental lines that disregard the individual in favor of the mass. A philanthropic person may be charitable and humanitarian, but has both the capacity and desire to be useful by giving large sums of money to specific causes...
So, the issue cuts back to "motive." In a characteristic sense, based on historical precedent and evidence, do I have reason to question the motives of billionaire "philanthropists"? Yes.
But, since I don't know Bill, I can't speak to his motives; perhaps he does genuinely believe in every charitable cause, for people, on his own moral principles, to which his foundation donates. In that case, he would properly be called a philanthropist...and also be deeply involved in a fundamental psychological paradigm shift.
At best, what I can say about Bill is that he (and his wife) have shown themselves to be humanitarian.