This article shows tablet usage statistics for January 2014 and January 2015. The article is about how much Samsung's usage has grown - so I think it's fair to say it's unbiased:
https://chitika.com/insights/2015/q1-tablet-update
This article shows iPad's high web usage early 2014:
http://www.cheatsheet.com/technolog...-web-traffic-in-north-america.html/?a=viewall
It's actually really difficult to find data on it though.
I've seen that Chitka data before, but unfortunately because it's just comparing use share
among tablets (and only those used to browse the web in North America, at that), it says exactly nothing at all about total installed base. 70% could be 7 out of ten tablets in use, or it could be 70 million out of 100 million. So it offers almost no useful information about how many of the iPads Apple has sold to date are still in use.
And it also, of course, says nothing at all about tablets that aren't being used to browse the web. Given the sales breakdown with Android, there simply
must be a lot of Android tablets floating around in North America that aren't being used to browse the web, or doing so very rarely. They can't
all be sitting in drawers.
So while better than nothing, it really doesn't answer the question "How many iPads are still in active use?", nor "Is the total number of iPads in use growing, flat, or falling?"
NetMarketShare has a few numbers that could be used to extract a rough web-only usage share of tablets, but most aren't available free; maybe one of the other web traffic aggregators has better data:
http://www.netmarketshare.com/report.aspx?qprid=17&qpct=3
Based on that screen resolution graph, though, given that iPads and maybe a few other tablets are probably the only devices still in use that report a 1024x768 resolution, it would imply that about 8% of all web traffic is on a tablet, and at least 70% of that is iPads. I have no idea what total number of phones and desktops are used on the web, so that still isn't a usable number, but it's something.
If
this site is correct that there are 3.366 billion internet users as of November, and about 8% of those are on iPad-sized tablets, and 70% of those are iPads, then that would mean an installed base of 188M iPads globally. With current cumulative iPad sales sitting at around 300M, that would mean roughly 60% of all iPads sold are still in active use on the web. Not at all sure how that compares to iPhone or desktop statistics for a product of similar age, and those numbers are unquestionably wrong, since they make a lot of incorrect assumptions and oversimplifications.