Galactic Readiness earned during multiplayer apparently decays at 3% per day. Does the readiness earned using the Mass Effect 3 Datapad app also decay? I haven't noticed any decay in the 3 days I've been using it, but the decay might be very small or take longer to kick in.
Galactic Readiness decays proportionally to the speed at which it was earned.
Readiness earned while in the ME3 Datapad iOS app is accrued slowly - fractions of a percentage per hour (depending on upgrades) and will expire at even smaller fractions of a percentage per hour.
Contrast this with readiness earned during multiplayer missions which can be accrued at several percentage points in a matter of minutes and will therefore expire at a much higher rate.
I believe this accounts for the different experiences of Galactic Readiness decay: hardcore multiplayer gamers will report a loss of 6% daily whereas those who play multiplayer little to none and rely on iOS apps to boost readiness will lose 3% or even 1% daily.
I definitely noticed Galactic Readiness decay on the days that I only used the datapad, so yes, it does decay. I am pretty sure there is no difference between Datapad readiness and readiness from other sources. You should be able to see the decay better within the datapad app because it shows readiness to .01%.