> ...via URL shortners, which is very unfortunate.

This is true for some tweets, yes. But most links are via Twitter's own shortener, which means shortened link rot is impossible, and the actual URL is shown to the user.

What is wrong with URL shorteners? (Sincere question)

It introduces a dependency on the shortening service, if the service vanishes the link breaks.

It allows the shortening service to track clicks on the link.

It obfuscates the target of the link.

I'd add that they allows evil persons (read SEO/Marketers) to hijack traffic by changing the target URLs.

I'm surprised that we haven't seen this yet.

The second a popular link shortening service goes belly up I'm sure someone somewhere will buy the domain and setup a program to redirect all the shortened links to an ad-ridden search page.

It happens all the time with small url shortener services. Also the owners sometimes uses cloaking to make the search engine bots get different redirection than normal users (for SEO reason).

It also creates unnecessary traffic, of course.

Not necessarily. Link shorteners can act like referrer guards, which is useful from a privacy standpoint.

No, they just shift the party that can see the referrer to whoever runs the link shortener.

Do CDNs use link shorteners to cache content in specific regions? Let's say A often shortens links and users B and C read them most of the times. So once user A shortens a new link the CDN (who would need to own the link shortener) could cache the content close to users B and C.

They hides the real location of the links and allows third parties (the url shortener owner and their partners) to track peoples.

Edit: like maxerickson said.