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.