I spend half the day watching YouTube videos. Luckily I have my own business so I can't fire myself or the company would go under.
I've only worked at one place where I know for a fact they monitored and blocked certain sites. It was one of the best jobs I had when I started there (small startup) and one of the worst by the time I left. They were super draconian. I worked in the marketing department and we basically had to fight to get YouTube access as we produced videos and needed to upload them etc. Luckily I was friends with the IT guy so I had access to everything (although it was still monitored).
This place got so bad when we were still in a small building where we rented the entire space. HR would lock the front door at 9am. So if you were even 5min late you would have to knock to have HR come scold you and open the door. The couple times it happened to me I would just call my friend to come open the door.
Also one time someone in the company accidentally sent an email containing some attachment with salaries or something confidential (don't remember exactly) and sent it to the whole company. They made us all leave the building while IT went to every single persons computer to delete the email and file. Plus if they somehow noticed you had downloaded the file you got a meeting with HR to ask you why you downloaded the file. That placed was whacked out towards the end.
Luckily never had anything like that anywhere else. In fact most my jobs have been super laid back. Maybe because they have been at more creative type companies. I can understand if you are paid to do some kind of labor job or something where you have to actively be working all the time. However in my case it's more like I get paid to do projects. As long as the project is done by the deadline fuck off! When I did more programming stuff sometimes you just don't have the solution to a problem you need to solve. Sitting at your desk in silence staring at the wall will not help you solve the problem.
Now if you're suppose to answer the phone or something and people end up waiting on hold because you're surfing FB then that's a problem.