This question already has an answer here:
I keep catching one of my team members (who I am managing) on youtube, social media and chat.
Our environment is quite relaxed in that sense - we all do this. Hence posting of the workplace right now. So the question is at what point should you put your foot down on the issue? I am a bit concerned it might be affecting his productivity in the respect that despite getting the work done it is taking him longer. At the same time I am giving him the benefit of doubt to let off some steam so to speak. I know from being a developer, you do spend some time idleing and thinking.