They can't sell devices that use an android-derived OS that isn't android compatible. As long as the phone could still run arbitrary APKs without modification, they're still within the terms of the OHA, and HTC can continue to sell official android devices. You can skin it up so it is unrecognizable, and you can lock it down so it only runs your App Store, you just can't change the fundamentals.

The trouble aliyun/acer ran into a while back was because aliyun actually changed the API, so that it couldn't run android apps. I can't imagine why Ali would have done that, and I can't imagine any reason why Facebook would purposefully cut themselves off from the android ecosystem. I'm sure HTC won't be running afoul of the OHA with this.

Facebook has been known to partner with Microsoft. It's possible that the phone will be full of Bing.

On the other hand, Microsoft is trying to promote their own phone OS, and this phone is clearly DOA if it doesn't run Android apps, but I can't see Microsoft liking the idea of promoting the Android ecosystem.

It's kind of funny actually. You could see how it could make sense for Microsoft and Facebook to work together and create some Facebook Phone running WP8, except they each probably think the other is going to fail and don't want to board a sinking ship.

I'm pretty sure they (Facebook) will lose the ability to call this an "Android" phone - and clearly they won't be entitled to the Google Suite of apps.

Further than that... I don't think there are any repercussions.