a) I disagree completely that there is any sort of implied promise not to compete with app developers. Even ignoring history on other platforms (where every single platform takes good ideas from apps and incorporates them) Facebook is too young a company for people to think they won't add something to their platform.
b) Even if there is an implicit promise, in this case Facebook acted very ethically in offering to buy him our when they decided to offer the same features. I can't imagine a better way to have handled that situation from Facebook's point of view.
Of course there's an implied promise. If platform operators openly stated "anything you develop can and will be used against you - by us" then no one in their right minds would develop on those platforms.
As you note, the trust involved is invariably abused, but always within limits - vaguely defined as they may be. What Dalton Caldwell is observing is that these parameters appear to be shifting.
That's FB's prerogative, and I don't object to their exercising it. But there were two ways to handle the change. The first was to buy Dalton out, shut down the project, but provide jobs for his team. The second was to do what they did with Instagram, preserving the product and the team, and letting things evolve from there.
It seems like going the latter route would actually encourage people who really care about products to build on FB's platform. The former route also provides incentives, but of a much lower grade, and only appealing to people who don't really care about the products they develop, and see the compensated shut-down / aqui-hire as their highest goal.
If platform operators openly stated "anything you develop can and will be used against you - by us" then no one in their right minds would develop on those platforms.
There is a difference between "using something against you" and a platform keeping their options open.
But, to quote Tim Bray from 2003:
They own the ground you’re building on, and if they decide they don’t like you, or they can do something better with the ground, you’re toast. They can ship their own product and give it away till you go bust, then start charging for it; and use secret APIs you can’t see; and they can break the published APIs you use. All of these things have historically been done by platform vendors.
My point is that no platform has ever behaved differently to how Facebook are now, and Facebook are at least offering money to companies they are planning on competing with.
People will continue to develop for these platforms, though, because they offer the incredible lure of money - they are were the users are, and where the users are the money is too.