It was announced on Facebook months ago several times, here in the forum too, by email, was tested by us volunteers, so what you are saying is a little wrong, just you were not aware.
Example : A More Reliable Response Rate Coming Soon – Share Your Thoughts on the Future “Response Rate”!
and it’s calculated easily : above 95% excellent rate, between 80 and 95, good, between 50 and 80 average, under 50 poor.