Last week I took a look at Instagram’s new short video feature, and waxed (somewhat) lyrical about why, in my opinion, it was no threat to Twitter’s Vine, which had rapidly expanded to 13 million users and long-eclipsed Instagram amongst shares on the Twitter network.

Turned out I might have been slightly off, at least in the near term. Since Instagram video’s June 20 launch, shares of Vine videos on Twitter have pretty much collapsed.

Check the chart below, which users data courtesy of those fine folks at Topsy. I’ve marked June 20 with a big red line.

Now, here’s the thing. While it’s certainly true that Vine shares on Twitter are down, and were already on the slide a touch before June 20, Instagram shares on Twitter have not really gone up. They’re essentially flat, which means that (a) Instagram users may be sharing videos, but they’re doing this instead of images – it’s not a net gain for Instagram, overall, and (b) accordingly, Vine’s fall may not be a direct correlation. Moreover, it’s very early days.

Still, it does seem to be a bit of a coincidence, doesn’t it? Vine best get those promised exciting new features out, and soon.

(Hat tip: Marketing Land.)