If Uber disappeared tomorrow, everyone would download another app and do the same journey in the same car with the same driver and the day after tomorrow would forget that Uber ever existed.

But today they have the biggest network of drivers, and until they do disappear (or get disrupted by something like self-driving cars), network effect ensures that they'll remain the default option.

Same can be said for Instagram or Twitter?

Yes and no. There’s no ongoing relationship between people on Uber, drivers don’t have passengers as “followers” or “friends” or vice versa. There’s no network effect really. And many users and many drivers have multiple apps anyway. The stickiness of Uber is incredibly low.

Yeah, and Uber's founder's name is Travis Kalanick. How does spitting out random facts about Uber at all address his comment that Uber doesn't have a sustainable competitive advantage.

Their sustainable advantage is in dominance. If they offer the most transportation options people check them first, and drivers flock to where the people are.

That's not how it actually works. Customers flip back and forth between Uber and competing services based on cost and availability. There's literally zero switching cost and no customer loyalty.