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.
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.
This is not evidence-based development strategy. This is a shot in the dark.