Here is the Mozcast chart:
Of course, most of this is because Google really has pushed hard to encourage webmasters and service providers to go HTTPS.
The interesting thing is only Google really knows the real percentage because a tool like RankRanger for their tracking set shows almost 98%. Well, 98% seems unrealistic. With RankRanger, they are tracking that 98% of page one SERPs have at least 1 HTTPS result, not that 98% of all results are HTTPS
Forum discussion at Twitter.