Facebook apps continue to proliferate, but there is a strong “long tail” effect to their adoption. Below is a graph of the number of users of the top 100 apps as of 4pm on June 26th according to appsaholic.
In aggregate, the top 100 apps have 63.5m users. Given that the 100th app, LOLcats, has 22k users, and even accounting for apps not yet in the directory, its likely that no more than 70m apps have been installed so far.
A recent WSJ article says:
Already all the activity has helped Facebook grow to 27 million active users from 24 million before the platform launch, with more than half using at least one of the new services, Facebook says.
So if there are 13.5m Facebook users with at least one app, then a Facebook user with at least one app has around 5 (~70m/13.5m) apps installed on average. There is likely a long tail distribution to this statistic as well, with many users with just one app installed (likely Top Friends given that it has 7m users) and some “application sluts” with 12 or more.
This suggests that there is still a lot of room for growth for apps on the Facebook platform.