What are some of the best growth/viral loops?

While researching Goodreads' growth strategy: https://uxdesign.cc/why-has-no-one-made-a-better-goodreads-dfc9cb9e149a

Ex: Dropbox t.ly/GRkk

This book comes to mind (have not read yet) https://www.goodreads.com/book/show/6912295-viral-loop