As a thought experiment, they probably helped the world global happiness more by burning $100B rather than just sitting on a massive cash hoard as so many tech companies enjoy doing. Sitting in their coffers it was doing nothing. At least this way, it went to a whole bunch of individuals, manufacturing startups, etc. And theoretically many of those recipients spent some of it.

I'm not saying it wasn't wasted spend, but velocity of money is a thing and maybe it's better off in the hands of the people who it was spent on instead of sitting in Zuck's war chest.

https://en.wikipedia.org/wiki/Velocity_of_money

Pakistan spent under $20bn to convert to about 25% solar. $100bn would probably have been enough for the whole country of 250 million people.

40 nuclear power plants.

Almost nothing. That is a few weeks of global medical R&D spend, spread over more than a decade.