Basically, Safari’s Shared Links section made you count as an active Twitter user, even if you never opened the view on iOS 7, as long as you had a Twitter account logged in in Settings. iOS 8 stops Twitter from counting you as an active user because it only fetches when Shared Links is opened. Twitter blames this change for 3 million users ‘leaving’.
However, I don’t think you can say that users have left Twitter because of iOS 8. These users should never have been counted as active, because they really weren’t. The way the Safari app worked before just made them classify as active in Twitter’s analytics. It was over-counting.
The new behaviour is a more accurate representation of how many people actually use the service. The three million weren’t “lost” — they should never have been included in the statistics at all.