Facebook ads allow you to target mobile device users like so:
Does this mean users who use Iphone regularly or users who are currently using Iphone to browse Facebook?
It's pretty self explanatory... All Mobile Devices by Operating allows you to target your Facebook ads on certain devices, for example if you have a iPhone/iPod/iPad app then you would advertise that on iOS, not Android or Windows. If you have research that tells you more people on Android are likely to buy into your services or products then you can target those users.... Generally you shouldn't need to select which devices unless you have 'PROVEN' stats that says otherwise.