There is a big gap between the phones on Gingerbread (Android API level 10) and post Ice Cream Sandwich (Android API level 15). Supporting older versions requires a lot of support libraries which themselves have some bugs. I kind of have a feeling that pre-ICS level phones is going to be in someways be like supporting IE6 down the road, so Facebook is just avoiding that.

It's also definitely not a 'handful' of phones. As of right now 55% of users are on the post-ICS versions. (http://developer.android.com/about/dashboards/index.html). The ecosystem has somewhat consolidated post ICS as well. There are now fewer phones each with more users which makes testing easier.

Depends on the OS -- Jellybean is different from Ice Cream Sandwich is different from Gingerbread is different from...

If you want something like this on all Android phones, it's more than just making an app to put in the Google Play store -- it's understanding the ins and outs of the different nuances of each OS...

Fast phones make sure the app works well, for one. It's like putting an app out for iPhone 5 only.