That percentage sounds about right to me. I've seen comments on blogs from ~10-15 years ago, that continue to have spam posted to them. The first 2-3 comments will be relevant, but comments 50-100 may have a single relevant comment along them, with a total of anywhere from 300-3000 comments. Older comments link mainly to blogs (*.WordPress.com) and such, while newer comments link to Facebook and Instagram.