Only 20% of iPhone Apps are actively used

"Today the App Store has passed the 100k apps available on the App Store (sept was officially announced 85k). This is a great news and the pace keeps growing.
But there is a bad news. Very few apps are enjoying the ride.
We have found out that actually 20k apps are actually used (meaning installed and kept on people's iPhone). This number may grow with our user base, but it is a clear indicator that only 20% of all apps are actually raising interest.
The second number which is more dramatic is that there is a very very very long tail of apps: The graph below gives you an indication of the rank of each app we scan (based on the number of installation) and the % of users that own them.

Click to expand

A very little number of apps makes it above 50% of iphone owners (easy to guess which one: facebook, shazam,...) it goes down VERY fast. The app rank 1000 is owned by less than 2% of the iPhone owners (1.76%) and it goes down very fast also."
Source: Blog post by AppsFire, 27th October 2009