Hao insists that a WeChat account is a private arena. He explains that Sun is the only member of academia among his WeChat "friends", and the rest are family and friends. "It is very different from Professor Sun's WeChat friends, who must include many peers," he said.

The difference between Weibo and WeChat, says a German who lives in China and maintains an active online profile, is that the former is like a public square and the latter a person's living room. Anyone can drop in and listen to what you have to say on your Weibo account, but only those who have friended you and been friended by you can sit in on your WeChat monologue.

But WeChat is still evolving while Weibo has pretty much matured, or as some would say, is going downhill. If you have only 10 friends on WeChat, yes, it is indeed a virtual living room. What if you have 1,000?

That is the number I have accumulated out of reluctance and I weed out many offers. I simply do not know the rules. When someone applies to friend me and gives his real name and identity, I always accept, taking it as a sign of sincerity.

If someone gives only the online handle and without a word of explanation, I usually ignore the request. I must have offended some that way, but I'm terrible with remembering names, especially online handles.

My logic is, if you want others to remember you, why wear a cloak of anonymity and mystery. Moreover, I suspect many of these requests are generated by software for the purpose of launching promotions.

My gray area is a request with an explanatory note that mentions my name but does not mention who he or she really is. I usually oblige, out of fear that I may slight someone. So, of the 1,000 WeChat friends I have, I know at most 300.

The result, I do not put up mutterings there the way I would on a dinner table of acquaintances - except when I want to plug a friend's book or my new play.

And I rarely follow others' posts unless I'm going through a night of insomnia. Anyway, I'm not comfortable reading others' diaries. I've found WeChat less informative than Weibo and it is an echo chamber with the destructive power of winnowing out diversity of opinions.

So, did the student step over the line of good manners? I believe he did. If his professor is in his living room, the student deriding and discrediting the elder's peers would make him squirm. The professor's reticence might be mistaken for his acquiescence to the student's opinion - if the posts are made public. And trust me, people repost their friends' WeChat mumblings all the time.

Call it "the selfie of internal monologue".