Interesting, I didn't ignore any of your points, but would be happy to be explicit in my response.

"It would be much less interesting as well... How many 'registered' users are actually active?"

This was why I responded with the 'daily active users', that is the metric of "How many users are actually active", you can argue that a better number is 'Monthly active users' which is a larger number, but the range is approximately 400M to 700M based on published data from Facebook and others.

" I know tons of people who don't use Facebook anymore, and some of them had multiple accounts. How many "registered" users use Facebook enough to see this?"

This is an anecdote, and an imprecise one at that so it doesn't technically count as a 'point'. How many people do you know? In total? How many people have you ever met in your entire life? If everyone single one of them dumped Facebook like yesterdays toast, what change would that make in Facebooks "registered but now inactive" list? 1,000? 10,000? 100,000?

"Most of the people I know log in occasionally to "check up on things", and never post."

Another anecdote, but a bit more actionable. You should consider polling them, did they even know that there was a vote going on? I check Facebook maybe once a month and I did, but I also read various tech news sites and its been covered by a number of them.

"How many people considered it worthwhile to vote at all? "

That is the question that is apparently being answered by this low turnout (and was my point). Here is my reasoning for that, I claim that people speak out about things they care about, I base that claim on vast quantity of statements that people make about things they care about and the relative lack of voluntary statements by people about things they do not care about, I further claim that folks who care about something work to preserve it if the effort required to do that work is proportional too, or less than they care about something, I base that claim on numerous examples of ideas or causes that have elicited widespread support such as Breast Cancer awareness, AIDS awareness, the funding of the Tesla museum at Wardenclyfe and a number of "This kid is dying send him a postcard" stories. I claim that clicking on a button sent via email is a 'low effort' activity, I got an email from Facebook (which I'm sure everyone else did) which had a link to go right to the voting page. I base my claim that it is low effort by the number of email click-thru products that are promoted. Finally, based on that line of reasoning I claim that this low amount of voting demonstrates that over 99% of the Facebook users do not care about this issue.

"How many people speak enough English to understand the entire issue?"

Do you have any information that Facebook did not provide this communication in any language other than English? How about as requested?

"How many people like expressing opinions at all?"

As I stated in my earlier line of reasoning, I believe that everyone will express an opinion, when that opinion is solicited, on a topic they care about. There is no mechanism by which I think Facebook can "force" people to share an opinion. That however would not invalidate a wide spread vote against this change.

"The incredibly low turnout is simply to be expected and is downright boring; but that half a million people bothered to vote no is interesting and newsworthy."

And this is where we disagree, but I'll be a bit more crisp in my disagreement. I claim that any action by less than 1% of the "active" users on Facebook is noise. There are cat pictures that got more 'like' clicks than people voted in this program. The thing that just got proven to Facebook's satisfaction is that changing the policy in this way will, at most, cause them to lose 1/2% of their users. That is really all they care about in this question.

If we do this, how many people will leave?

They got their answer. Practically nobody (well certainly no more than join or leave at random on any given month).

My original point is that "Facebook asks if anyone cares if they change their policy, answer is No." would be a boring headline for TechCrunch. Wouldn't get the clicks and the reads. Saying "Less than 1% of Facebook users care about privacy", same result. Not news.

Now they could have run with a lede that said "Facebook proves that people who care about privacy no longer use Facebook." But really they were whining that this wasn't as big a story as they wanted it to be, and to get people to read that whine they tried to 'pump up' the headline with "1/2 a million users ..." which sounds like a lot, when it really isn't in the context of Facebook's user population.