I was just reading a rather typical AAAC thread about this, which inevitably is dominated by people suggesting that Z'berg's choice of clothes was a deliberate attempt to show disrespect to the suits, or profile himself as a maverick or whatever.
It strikes me that it is just as likely that he cares very little about clothes and has been wearing the same sort of clothes since he stopped wearing nappies and therefore spent precisely zero seconds wondering what to wear and just put on whatever happened to be in his wardrobe, or even in a pile on his presumably expensive floor. The fact is, he dresses now as a billionaire just like he did when he was a student and when he was starting out pitching FB to venture capital.
We can lament that people do have this attitude to clothes, but about 80+% of people do. It doesn't mean they are making a statement. Or even tthat they are Bad People (as this board has often established, genocidal maniac dictators are often more interested in clothes than non-psychotic leaders: compare Colonel Qadhafi (dec'd) with Jimmy Carter).
I found myself a couple of days ago talking to someone (a perfectly reasonable person in all respects) who was utterly dumbfounded at the idea that I might wear a shirt at weekends. You know, a shirt with a collar and buttons all the way down the front. Not a t-shirt. I don't think he'dd have been more astounded if I'd said that the world was secretly run by budgerigars.
People like that are not members of an organised insurgency against style or good taste, who are plotting a massive DDOS attack on this forum. They are just ordinary people.