With the trend of larger and larger mobiles, and the fact that 98.3% of facebook users log into the mobile, while only 1.7% use laptop/desktop exclusively, there is a lesson to be learnt here:
Like it or not, the vertical online photography and vertical videos are here to reign.
(not everybody care to twist their phone in landscape mode - far, far from it.)
Vertical does not just apply to personal sharing, commercial giants like Coca Cola and BMW know exactly how their customers behave from market feedback - and they have turned to vertical online campaigns.
So if you only photograph for yourself, and not to share with others, any format would do.
But if you are competing for customers *online* or do share photographs for fun *online* - can you afford to not shoot vertically, when so many has left their laptop behind, and watching/working vertically on their phablets?
I guess some airheads will say, nah, my art is so great that people will turn their mobiles to landscape mode when they watch my masterpieces, or Im really an modern Ansel, my art is too great for mobile, it can only be printed large and seen in fine-art exhibitions.
To them i would say, sure buddy, sure...
Some platforms like instagram are so restrictive that they don't even allow a vertical image beyond 4x5 ratio. They only want squares and the vast majority of users on instagram post squares.
Oddly, instagram allows wide screen images, I think to 16x9 ratio, but no wider. The problem is the instagram app does not allow rotating the phone, so, that's dumb!
Head over to Youtube and you'll find that 99% of the videos are still widescreen orientation.
100% of movies and TV are still produced in widescreen format.
You can buy a 65 inch TV, a Roku, and Netflix for a year for less money than the latest iPhone costs.