Steven Levy, world famous author, wrote on Back Channel a bit about RankBrain, here is the quote:

So back when they first announced it, it was used in about 15% of queries and now it is used in “every query.” And the impact is felt in “a lot of queries” but not every one he reported Google saying.

Danny Sullivan broke this out into a story last week and then updated his useful FAQ on RankBrain.

The weird part, I still don’t see how RankBrain is a ranking signal over a query interpreter or modifier.

We reported last week that RankBrain can’t be optimized for and there is no RankBrain score.

In any event, it is just confusing how Google has positioned RankBrain as a ranking signal in my opinion. Other than that, RankBrain is heavily used in search, whatever that means.

Forum discussion at Twitter.