You're bang on the money with API's in general and I'm sure facebook etc have a more stabilized API due to it's popularity - it'd be bedlam if it changed too much.

But I am under the distinct impression that it's a common bane to have web api's changed on you, abstraction through some form of secondary interface is a wise precaution. Web tech is changing very rapidly, another year or two and we could see things like the Facebook API slowly morph into different web incarnations, greater adoption of HTML5 etc new security approaches etc

It depends on how much freedom you have to update any binaries etc for games/apps you make.

I concede that as a precaution it's merits might very well be outweighed by the amount of work involved, I'm just thinking back to all the times in my coding life where a bit of abstraction could of saved me a lot of work.