I had this problem. Solution was to install a Chrome extension called "StayFocusd" and really lock down what it allows me to look at. Only sites that are work related: mail, all the local web consoles (Jenkins) that I use, stackoverflow and friends, and websites containing "Cpp" or "python", and a few other keywords.

Wikipedia for entertainment. It's complex enough that it's like reading a school textbook: your mind gets tired from reading about the war of the Spanish succession, and decides to go back to work.

Everything else I give 3 minutes a day to, within the working day. So if I show up early or work late, I can browse a bit, but the 3 minutes can be used on random sites I need (say someone emails me some form I have to fill out).

I also use "TimeYourWeb" to show me what I looked at, and it's much better since I did this. Normally I didn't waste all day reading, but now and again I'd run into some interesting long-form article with associated links.

Now you might think 3 minutes is too little and I'd then cheat and go on Safari or my phone. Which I do, especially for Facebook. But I generally use FB to tell my friends I'm still alive by liking their articles. The articles linked from there are either things I've already read because they're all over the news that day, or not very interesting.

I also find the leap of having to copy-paste a link to Safari is enough to stop me doing it for most things, when I would have just clicked through on my main browser.