If I were Facebook, I'd just avoid Google entirely and use alternative apps for everything. Amazon Appstore, Facebook Messaging for email, etc.
The source to Android is made public at or around the time the final version ships but until that point only select partners have access to the software. It's this approach that Google are going to strengthen - essentially if you don't obey their rules you won't be a select partner and you won't get early access to the new releases.
What this means is that if you don't obey Google's rules you'll be getting the software two or three months later than your competitors (and without all the support they will be getting), likely receiving it around the time they're actually bringing products to market. That's a pretty big competitive disadvantage if you're aiming to compete at anything close to the top end of the market.
So basically while you can use Android as you wish, if you don't obey Google's rules then you won't be able to do so in a way that allows you to be competitive.
The cynic in me says that this is getting mighty close to being open in name only.
Also, while there have been some performance optimizations in the latest builds of Android, from a feature perspective there hasn't been that much (NFC is the biggest one in Gingerbread). So even working on the previous release doesn't seem like a huge deal to me.
Even if that is true if you're Facebook or Amazon and you're looking to differentiate yourself in that way it's one thing. If you're a handset manufacturer looking to make a kick-ass Android phone it's another entirely and there it would be a big deal.
You may be right for small point releases but when they revise the interface or introduce significant new features or something else which people will notice and want, it's going to put you at a disadvantage.
And because you don't have access to the new version you don't know whether it's going to be a significant release or not. You may be competitive, you may not, you simply don't know.
I honestly don't see how that doesn't put a company at a disadvantage - it's certainly not a level playing field and it's certainly not a situation I'd happily put myself in.
How much of a big deal this is would depend on what market segment they're going after, but mapping and location services are awfully handy across the board, and available on pretty cheap phones at this point.
Maybe Google didn't really have the clout to exercise control until now, but I'm really glad it's finally happening.
What I hope this means is that Google will control everything on the OS level, and allow customizations by the manufacturers and carriers only on a skinning and app level. In a perfect world they would also enforce access to bootloaders, to allow easy rom changes.