Because -- given the tech talent crunch -- tech companies are competing for talent. One way they do that is by providing more aggressive benefits. This means that as large name-brand companies change their benefits (such as parental leave) for the better, other companies are likely to follow.
And I speak here as someone who works at Netflix, which went public with "do what makes sense for you and we'll cover you for a year after your child's birth or adoption" a few months ago.
In short: Facebook doing it will have a positive impact on the benefits arms race which non-Facebook employees will benefit from.