Somebody asked me this question earlier. So I went and counted; it was under 10% (sample of around 100).
I find it interesting that whenever there's a post on HN that's supportive of Google+, the community collectively reacts with extreme skepticism. Just... interesting, that's all.
I'm not sure I read pork's comment as skepticism, necessarily. Rather, I'm more curious as to why, if such a forum is an unmet need, an alternative hasn't managed to gain traction while Google+, at least in the author's opinion, has.
I wonder if Google, having a fair number of CS researchers, might be able to get past critical mass / escape velocity / choice analogy by simply starting with its internal users and growing organically from there. You have to have something for the community to nucleate around.
I suspect it has to do with farmville. I can't say for sure, but I hazard to guess that CS researchers don't play farmville and probably prefer not to mingle with those who do.
