I think you forgot the part where Android fragmentation leaned more towards the 3 year old version. 2.3 is still huge. If Android had simply evolved fast and all the new phones had the new Android the problem wouldn't be so bad.
I don't know about "Dropping Fast" - it's slowly dropping away, even after they changed how they gathered statistics to only count devices visiting the android store.
And really, I'm not entirely shocked that a Twitter-owned service is getting more sharing on Twitter compared with a service that gets a reasonable amount of obstruction from Twitter.
That doesn't mean people will riot if it isn't available on their device. If you've got Android 2.x and can't get Vine, you likely don't really care as you haven't ever used Vine.
I think you forgot the parts where even devices with a 3 year old OS [1] regularly get updates of many APIs that have been (sensibly) decoupled from the OS, have access to substantial compatibility libraries and so on.
[1] By which I mean Froyo, Gingerbread doesn't hit its third birthday until December (at the earliest).
