Even though Facebook for Android may have higher monthly active users, the iOS user base from a marketing perspective is a more attractive demographic to target nascent mobile revenue opportunities at. Soccer moms almost exclusively use iPhones.

Despite more uniques coming from Android, maybe iOS user show more engagement.

I believe that's what the statistics say. eg, something from a Google search just now:

http://www.netmarketshare.com/operating-system-market-share....

This again? Really? The main improvement in this version is that it was rewritten for iOS, it didn't need to be rewritten for Android. Whether or not you have more marketshare doesn't mean you need an update for a platform you're not on.

Having used the android version from since it first came out:

It definitely needs to be rewritten for Android.

Much easier to optimize for three very similar phones versus hundreds (thousands?) of Android phones?

Android has the market share, iOS has the usage share. Lots of people get android phones who have no desire to use it as a smartphone, they just get a free phone that happens to be android. iPhone users actually use their iPhones for Facebook and web browsing and other smartphone tasks.

Possibly because most of the employees use iOS...