Shortest is a very old, in fact one of the oldest shorteners, with an outdated monetisation strategy, that does not help with its success now or in the future. Currently, it has also experienced a ban on paypal, resulting in many publishers assuming that this Polish company is a scam. Lets check out their 1 ad page:
Interstitial Ad - $0.15 (it is from their own advertisers)
Fullscreen Ad - $0.5 (it is from their own advertisers)
This site earns a mere $0.65 per 1000 worldwide views, however their rates for this traffic is about $0.325, so they pocket 50% of earnings.