Does using or watching YouTube break a bhikkhu's rules? Would using YouTube or other social media, where one is exposed to music or forced advertising/entertainment, be breaking the rules for monks?


  "the watching of entertainments are stumbling blocks"


This makes watching any of YouTube (Number 7 of the eight precepts) an area of 
concern.
It is my personal experience that frequent games, movies, TV, YouTube, smart phone use and browsing definitely reduce one's mindfulness.

This is a  fundamental question about all entertainment - why must entertainment be eschewed? The nature of boredom is that it arises out of a mind that isn't at ease with itself. The nature of thought at the moment a bored mind arises is to gravitate towards the outside world - towards sense pleasures. 

Distracted thought gets reinforced with every application of distraction, so much so, we live in a world where hardly anyone reads long pieces of text voluntarily and people find even Twitter too difficult to concentrate on. 

The nature of boredom is that it gets excited by violence rather than peace. It reinforces duality, of the entertainer and the entertained. It is thus a quality of mind to be safe guarded against.

YouTube itself is just a tool, it is neither good nor bad, it is the nature of the mind that watches YouTube that is the determining factor. There are a good many dhamma talks on YouTube for example.