It works for me b/c the bulk of my "friends" on facebook are also devs, so they get it. Plus, while GitHub does have a newsfeed, it's always an overload of information.. I think this makes it easier to keep up with friends coding activity.
I assume my non-developer friends don't understand it, so they just ignore it.
The beauty of how Facebook bubbles up relevant information is what makes it work best. My mom may have seen it once, but probably won't see it much anymore.
Yes, there are definitely some people who only use Facebook for friends and family, so it makes no sense for 'push commit' to show up in Facebook.
On the other hand, it can make following what your friends are doing without having to sift through Github's feed... which is a mess if you ever try to find anything and watch a few projects that have lots of activity.
Useful for those who have strong overlap between people who would care about their commits and friends on Facebook that would give a shit.