I think there's a difference between usage and github stars. I generally don't really star many Big projects like angular2 or react, since there's so much notification spam on it. I star smaller things which are likely to have less-frequent updates, but each update could well be a big deal (relative to the size of the project).

I was surprised also, the number of stars it has should put it into 3rd place, so I guess it had a bunch before 2016 started already, even though it only went out of beta in late 2016.

Keep in mind that this isn't absolute stars, but stars received last year. Many people who are happily using Angular might have already starred it earlier.