Don't worry about 'rules of thumb', and especially don't worry about what other people are doing or not doing.
If my boss expects me to have my nose to the grindstone for only 75% of the time I'm on site at my workplace and I tell you that as a "rule of thumb" then this is no help to you if your boss has a different rule.
It's no use to assume that what your boss allows another person to do is an acceptable 'rule of thumb'. Your boss might be a jerk who treats people unequally, or they might have perfectly good reasons that they have not discussed with you because they're none of your business for treating the other employee differently.
And 'being on facebook' and 'reading articles'. Depends on what that means. If I'm on facebook messaging a friend who works in a similar job to say "have you seen this weird bug in $foo because it's kicking our ass here. Got any ideas what to do?" then isn't that working?
If he's "reading an article" about a java vulnerability and some of his code happens to be in Java then again, isn't that working?
If he works best by taking frequent mental breaks away from a problem when he's stuck rather than trying to grit his teeth and charge through it, and he's therefore more productive that way then... well maybe during those breaks he isn't working, but he might be more productive overall, and if the manager is happy with them then that's what really matters.
I'm one of the people who needs to take quick 'breaks' from a problem I'm stuck on to re-adjust my sights... or churn it over in the back of my mind while the front does something less engaging. I sometimes do this by visiting stack exchange where I talk to people and learn things that are useful to my job, or help others and build up goodwill for when I/my employer needs help with something.
If you walked past my desk while I was doing this it might not look like I was working. But I'm out the door bang on time most days and still well ahead of what's expected of me by the end of the day.