I'm a Facebook user. I'm also an iPhone user. I'm also a bit lazy about updates. So having the Facebook app on the iPhone seems like a good idea. But there's one interface element in the application that frustrates me and makes me prone to not want to use it at all.
If you use your iPhone to email, you're used to sending it using the SEND control in the upper right hand corner of a message. It's a good place to be for right-handed people, as it's easy for your right thumb to jump right there. I send emails all the time from my phone, so I'm really used to this behavior.
But enter the Facebook mobile application. The most frequent thing I'm likely to do there is to update my status. ("OMG I'm totally writing a journal article, AFK LOL") But someone at Facebook decided that for this screen, the SEND control would be in the lower right hand corner of the screen. Which wouldn't be so bad except in its place they put a CANCEL button in the upper right hand corner. So after going through the motions of writing, reviewing, and doing any corrections on my pithy and charming status update, my brain says, "Awsome, ship that sucker." Thumb snaps up, and <expletive deleted>. This might be mitigated with an "Are you sure?" dialog box or some clever recovery method, but no, it's just lost.
What's worse is that the app isn't consistent even unto itself. Open up a new message in the Inbox, and what's in the upper right hand corner? A SEND control. Where it should be. So even if you were a loyal Facebook user, you'd get caught in this same, confusing situation.
By the time you read this, the application may be updated and this little problem fixed. But it's a nice reminder that internal and external consistency with conventions are vital to keeping your users happy and meeting their goals through your software.
Update: An early editor asked me to check if the app had been updated, and in fact, it has and this issue has resolved. Still, the lesson remains.