Interesting chart by the business insider which shows that it is not very useful (of course relatively) to write Facebook apps.  However, things have to still be kept in perspective. The 10% is still from a user base of 600Million+ users. Hence at any instant of time, you will have more than 60million+ users on facebook apps, which is still pretty neat.

That is what I think. :)

Time spent by a regular Facebook user.