It's the 1% Rule.
Only 1-10% of users who visit the site actually create an account. Of these 1-10%, only about 10% of them use the BBS at all and 1% use it frequently. Only about 1% actually submit content and even less create quality content.
These numbers are just rough estimates, but the 1% rule is a good indicator of why you don't see many users posting on the BBS or submitting content. You simply don't see the outcome of most of Newgrounds' traffic because few sign up, fewer post on the BBS, and even fewer contribute quality content.