I get to choose when I go to HN. I just closed a bug I'd spent the better part of the week on, I'm fine with relaxing.
In an open office I lacked the ability to choose interruptions. I still remember Dipesh coming over, slamming his hands down on my shoulders, to ask "what's up buddy". You could hear the stack falling out of my brain. He just wanted to chat.
Have you ever considered that FB, HN, etc are the way people cope with the extra stress, distraction, and low level anxiety of working in an open plan office?
When I work from home I don't waste time on these distractions like I do when I'm in the office.
I have considered that, but I don't buy it for one moment. Open plan offices have many advantages, you just need to hire people that are mature enough to deal with it. People who can self regulate. I don't mind some time spent on FB and HN etc, I think the latter is actually a net-positive, there are technologies that osmotically people become familiar with through reading about people's experiences. But FB is 100% a waste of time. Corporate IT don't block it, they just measure it and provide management reports on it's usage.