Here's the FA statement:
http://www.thefa.com/news/governance/2015/feb/adrian-wrongful-dismissal-claim-upheld
Which to me confirms it was withdrawn solely because they decided it wasn't a goal scoring opportunity, not that it wasn't an infringement by the keeper, nor deemed to have been preceded by a foul by Mane. If Adrian hadn't dived onto the ball then surely Mane would have had a goal scoring opportunity?
Edit: the common consensus amongst West Ham fans seems to be that it couldn't have been an "obvious goal scoring opportunity" as they were both moving away from the goal, not towards it, which is a fair point I guess. That said, it wouldn't have taken a lot for Mane to nick the ball away from Adrian's feet (if he hadn't dived on top of it) and slipped it into the goal with a nimble 90 degree spin.