Why are numbers needed?
A nebulous question. But perhaps we can think about it logically.
Apple provides a direct feedback system to pipe in feedback, snapshot data of a situation, as well as context to a perceived bug to their developers. Compare that to an online forum, from a different company that typically contains only one of those items, the complaint, often lacking context and other helpful features.
::Edit::
Sorry completely missed the context of the question. I don’t have direct numbers, but listening to podcasts, discussing issues with people on here, and working with friends, not many even know about the feedback system, or, think it takes too much time to use it.
Perhaps it is widely used, but I believe (again no facts) people chose to vent on forums, Twitter, or YouTube comments, more than using an actual feedback system.