Anecdotally, when I was a young adult I was a volunteer moderator for a large forum. We got reports of CSAM several times a month and had a process for escalating and reporting it to the FBI IC3 - we retained a lot of information about the users that posted it.
One of the administrators of the website mentioned to me that over the years since the inception of the forum, they'd reported almost a thousand incidents of CSAM distribution - and the FBI followed up with them to get information less than 10 times in total.