Doesn't just have to be FB. All platforms have some benefit, and all unions would do well to embrace each.
IMO, the benefit for labor is increased "reach." Both of our employers can get to every employee instantly, and we're trying to counter that with...flyers?
And while you and I may be used to getting info that way (or going to the hall), the 24-year-old on your gate isn't. That's not good or bad; that's just reality.
There's also the benefit of getting a clear/consistent message out every time. NYer alluded to the lack of clear messaging further up this thread, and how it's causing some undue hurdles.