1. It's a drop in the bucket compared to what governments spend on a regular basis trying to solve these problems. (Bill Gates has said as much.)
2. Private charity by the billionaire class is not a scalable solution. Historically most social advancement has happened through popular organization and government programs, not charity.
Silicon Valley itself is a product of government spending. The Internet and thus Facebook wouldn't exist without billions of taxpayer investment in early stage high risk research and procurement via DARPA and other government agencies. That continues today (just a couple examples: Siri and autonomous vehicles).
If we are serious about accomplishing social change and "long term investments over 25, 50 or even 100 years," the answers lie in greater government investment in these areas. Just like Silicon Valley. And that means all Silicon Valley companies should be paying back to the government just as they would an early stage investor. Not as a "noble choice" but as an obligation. (Currently they get the core tech pretty much for free.) That would drum up an order of magnitude more funding for much-needed social projects.
I'd like to hear more Silicon Valley CEOs talk about that.