>>>"But is any particular billionaire (or any other particular individual at all) somehow obligated to fix the homelessness problem? No."

As you get wealthier, your ability to affect society expands. I'd argue that, in the same way one can feel responsible for members of your family, society is your larger family. Humanity, an even larger one. Perhaps a wealthier individual should feel as though more of society is their 'family' as he/she gets wealthier — and treat them as kin.

Instead, what you often find is self-involved individuals becoming more and more distant from their society as they get wealthier (large households with gigantic yards, vacation homes, remote travel).