Ouo.io is the worlds best alexa ranked url shortener, so it must be doing something right. And I think it is. Great tactics, balanced rates, modern design, high earning ad pages. If I were to pick the most successful and deservant shortener it would be ouo. Let's check out their ad pages now!
Rotation Banner (this means a different ad network with same earnings as others appears everytime) - $0.4 (CPM/CPC/RPM (random))
Floating video ad from VDOAI - $0.4 (CPM)
3 Rotating PopUps (they are from random ad networks, but always same earnings) - $0.8
!Fake download button! - $0.3
Test Banner - $0.3
Altogether, this masterpiece of a shortener amazingly earns $3 from such a minimal amount of ads.
RPM - Earnings from Clicks and CPM in ad networks altogether per 1000 impressions.
CPM - Earnings from just views and no clicks per 1000 impressions
CPC - Earnings for 1 click
CPA - Earnings when someone completes an action such as a signup.