Because other people are taking care of sanitation related problems. Why can't we tackle more than one issue? Throwing money at one problem, solving it, and moving on to another one isn't necessarily the most efficient way to solve those problems.
Yes, of course you want to direct your efforts at whatever will bring the largest marginal benefit -- and you're right that that can change if other people are already working on the absolute best things. However, that doesn't mean that all the other options are equally good! Would Facebook's money be better spent on internet access, or by being one among many working on sanitation? It's not a forgone conclusion, and it would be nice to see people be more data-driven about how they spend their charity money.
In practice, the internet thing is probably Facebook's only option, since it's the only one they can convince their investors to back. This isn't really charity; they intend to make a viable business. I really like that aspect of the plan. Charity comes and goes, but businesses tend to stick around when they're making money.
