Quora seems to me to be the intellectual equivalent of Muscle Beach. Sharing how clever you are is its social object. And it works, because this taps deeply into a type of male behaviour that has changed little since men first gathered in the Agora of ancient Athens.
To summarise, it's a place where men ask other men questions, which get answered by more men, and are then rated by men.
By the way, this doesn't make it un-valuable. Let's face it, those Greek guys provided a foundation for the whole of Western thought. And I can see that sometimes I will want to throw a question into this seething cauldron of competing clever-clogs. The answers from experts are often truly interesting, although balancing the good stuff with the noise produced by millions of morons will be a difficult task. The acid test, I guess, will be whether the expert people stay engaged with Quora. Without them, it's just a bunch of blokes gobbing off. Useful? Perhaps. But will I be going there 10 times a day? Perhaps not.
On the other hand, Instagram's charm is the simple pre-verbal delight of sharing something beautiful by showing it to other people. It's even more primal than Quora: more like visiting the cave paintings of Lascaux and Altamira than hanging out with the chaps shooting the breeze. It's about a shared wonder for the world. It's more generous, I think. The subject matter, certainly of the people I am following on Instagram, is mainly 'the things I love or find to be beautiful': meals/food, good times/friends, family/kids, places. It brightens my day, and I am visiting it at least 10 times a day. And I feel really connected to the people in the network. It's weird, and surprising, but in a good way.
Quora and Instagram cater for ancient needs in new ways. Showing off and sharing are two sides of the same coin, but one is mercenary and the other is about generosity. I know which one I find more engaging on a social level.