I see them targeting the market of businesses too small to run their own communication infrastructure. The people using google docs and hangouts because it's already there. Or Office 365 because they can't/don't want to host internally.
Why would anyone already running their own communication want internal Facebook? Because email lists aren't hip enough?
I don't see it as an email replacement. I see it as truer to it's name. A Facebook for your company.
Let's say you are going to do some work with Molly from the Customer Support department. You don't really know much about her, so you bring her up on the company Facebook, and you find out that you both went to the same university around the same time. You also find out that she has experience in testing. You also see a picture of her, so that you don't have to go walking around customer service area of the office asking if anyone knows where Molly is until she finally overhears you and introduces herself.
Is any of this stuff hard to set up internally? Not really, but someone has to prioritize it, sort through existing options and pick one, or develop it in house, there needs to be support, and maintenance. All barriers to implementing something that can help foster a more open and collaborative culture.
Why would anyone already running their own communication want internal Facebook? Because email lists aren't hip enough?