Also comparing Uber to Facebook is quite silly too. Facebook had a lot more users than Uber but they are also different types of users. One is a free user with hope for targeted advertisement (in which the advertiser gains/loses money) whereas the other is paid users/riders (but in this case Uber has been losing money each ride and I don't see it changing). Also advertisement has huge margins with not much cost from Facebook's pov but ride sharing logistics is completely different with very little margin where the driver needs to get paid. Drivers are already complaining that they are not getting paid enough for rides and asking Uber to take lesser cut of the ride. So Uber can't raise their cut over their current 20-25%. Facebook's advertisement business doesn't have this problem. Nor does Amazon as they can make their logistics more efficient.
Amazon is able to lower it's cost with economies of scale but I don't see Uber being able to achieve that as their costs only increase as they scale as they lose money on every ride. The only way I can see them become profitable is if and when they achieve driverless cars, which they are betting huge on, but that's a long way to go. Uber is betting on driverless cars to lower the cost per mile. But I think Tesla will beat them to market much much before them considering Tesla already has the tech. Elon even hinted on the Tesla taxi fleet in the next couple years.
Now I realize Elon's got poor judgement on timing but I still think they will be able to achieve that much earlier than Uber. And if they do, I honestly don't see Uber & Lyft surviving.