I see a common obstacle for Microsoft, Google and Facebook's entrances into AR. I expect they will all struggle to incentivize enough developers to build a vibrant ecosystem.
Google's Project Tango has been around since 2014, and it is almost universally considered a flop. Project Tango only works on specialized hardware that relatively few people own, and because of that, the software landscape is bleak.
Microsoft's HoloLens was announced in early 2015, and it's really quite impressive by all accounts. But it's not really a consumer product. The HoloLens starts at $3,000 and is marketed as a developer edition. At this point, they have too few users to truly attract developers in large numbers. Obviously, this could change dramatically if they announce a truly revolutionary consumer device.
Facebook's AR Studio is only a few months older than AR Kit, and Facebook has 2 billion users. But at this point, Facebook isn't offering a way for developers to monetize their AR Studio creations. This means it will be filled with AR "apps" that are essentially ads for companies that monetize in other ways.
In the fall, Apple will update their iPhone line and hundreds of millions of iOS devices being used today will be updated to iOS 11 and capable of running ARKit apps. This is serious incentive, and I expect to see a cascade of AR-enabled apps in the App Store at the end of the year.
This is exactly why Apple's stubborn approach to top-to-bottom proprietary/integrated technology pays off.
While everyone else gets bragging rights for beating Apple to market, Apple focuses on becoming first to mainstream adoption.