Within friend groups where there are no formally established laws or codes of conduct, there are certain implicit standards of behavior people are expected to abide by, and part of why they do is because of the threat of social ostracism.
In a broader scope, in ancient societies social ostracism was often used to deal with those who violated the norms of society; in Athens ostracism was an official punishment.
I'm not saying we can or should literally expel billionaires who don't put their liquid wealth towards philanthropy, but far too often we focus on formal, legal means of compelling the super wealthy to part with their money, such as through taxes that are easily avoided.
Social pressure put upon the super wealthy using tools like this could be more effective than people expect. There is the threat of social ostracism, but also the reward of social approval granted to the patrons of philanthropy.