This is so hard to quantify. A developer ecosystem around your product helps build engagement and lock-in. It provides free testing ground for experiments with your data (for better or worse).
A lot of people look at this as a "gift" to the developer community, but the relationship is symbiotic.
Now, given where Facebook is, am I surprised? Hell no. Billions of investors' dollars went away (temporarily, surely) almost overnight[1]. Their reputation took a hit. They needed to do ... something, anything. I get that.
But they didn't take a smart approach, likely because of the time constraint. The smart approach is lock down the parts of the graph that have the largest vectors for privacy invasion. Instead, they just hit the STOP button and are apparently calling it a day. In fact, there are some conflicting reports on some endpoints that officially will be up for review but privately have been discussed as permanently deprecated. I expect a walled garden.
We saw this with Twitter for different reasons. The lesson - as a developer/entrepreneur - should be: never make another company a mission critical component of your own. If there's a rug to be pulled out from under you, one day it will be.
[1] https://techcrunch.com/2018/03/20/facebook-has-lost-60-billi...
Maybe, maybe not.
Let's say tomorrow InstaFaceSpace launches and gets a bunch of hype. You know, it's got shiny fonts and your mom's not a user yet.
There's a migration, just as there was from Friendster->MySpace->Facebook. InstaFaceSpace is open for developers, giving them the tools they need to make and monetize applications on top of their platform. Stop me if you've heard this one before ...
Let's say this is embraced and there's some tangible migration. Apps are being built on InstaFaceSpace - cool apps, fun apps, secure and privacy-ensured apps.
Now Facebook realizes they need to have the platform they threw away and its ecosystem to retain users. Now what? Can they say "hey come back we've got all this new stuff and these hot new endpoints?" Developers won't go back.
It leaves Facebook vulnerable. Perhaps not as vulnerable as the PR nightmare they're going through, but there are scenarios where it comes close.
I'd argue it was the apps (and the publishers) that led to the decrease of organic posting in Facebook. Instead of seeing posts from your friends, now you see viral articles, and app-related statuses.
I think that early on that API was definitely making them more money, but the balance has shifted. I suspect they kept the API around as long as they did to avoid a sudden PR disaster when they removed it (like twitter). The congressional hearing and all the recent news has served to lessen that cost and may actually end up being a boon for them.
If anything, it was preventing advertisers from spending more money, or using their own tools like Facebook Insights.
Now that Cambridge happened, they're using this golden opportunity to shutdown the other API endpoints, so that they can keep more of the data, and advertisers will have to spend more money.
If I'm being emphathetic, I'll say I'd even do the same thing if I was in Facebook's position right now.