We've rolled out recently private SO in my company and it's been generally very well accepted. It proved itself useful in answering company specific questions. I think it is also perhaps down to that inside the company we have better control on overal culture of people?
But certainly there was similar patterns emerging - higher ranked colleagues tend to answer more questions, leaving less room for other colleagues to get points on answering. But then the question is, does it matter? In company the goal is to share knowledge, it doesn't matter who has points or not. If the knowledge is shared that's success.
We're a place where coders share, stay up-to-date and grow their careers.
We strive for transparency and don't collect excess data.