Probably about the same percentage as Box Brownie pictures from 100 years ago.
It's true... it's all a feed, a cycle ... no one will care in 100 years, they will be part of the "dark web" there, but inaccessible to most, except the rarely good ones (like that hurricane that engulfed NYC that someone caught with their cell phone on an airplane and instagrammed it... and it went viral, and was on the news... that might still exist...
The Wayback Machine usually only archives HTML and plaintext but often does not copy pictures, movies and other multimedia from the websites that it crawls. Further it doesn't copy entire sites. Usually only the first few pages. When it does copy, it only copies the changes from one time period to the next. Therefore, dynamic websites like web forums, blogs or social media sites can not be copied reliably all of the time.
Wayback is good to let you know that a site existed at a particular point in time but its not always a reliable record.