I'd like to target a message to my Facebook friends who use iOS, and separately, Android. If I'm going to recommend them a particular app, for example, I'd like to ping only those who have the right OS. Presumably Facebook has this information. Is there a way for me to retrieve it?
I do not think Facebook would expose this kind of information on their API. What you could do is use Parse and its Installation class. By doing so, you will get specific device information of each of your users. Then you can use the Parse Push SDK to target specific users based on this information. As an added bonus, Parse also offers Facebook integration, so it might be the tool you are looking for.