From what I understand, piracy on iOS isn't an issue. The only place where it's really significant is China. And there are two points to keep in mind there:
1. Stopping piracy of a single app in China is very unlikely to result in increased sales of that app.
Well, what makes something an issue? There is certainly piracy on iOS... people jailbreak their phones and then pass around ipa files on the standard piracy sites.
I'd say it is similar to piracy on game consoles. It exists but you need to do something unusual to enable it, so most people don't do it.
Right. It happens, but that's not the same as being an issue. In terms of a developer efficiently using their time, chasing after the <5% of your users who pirated your app, and who in all likelihood only heard of your app because it was one of the ones they found on a piracy site, is not a good strategy.
1. Stopping piracy of a single app in China is very unlikely to result in increased sales of that app.
2. There's no Twitter in China.