I tried to extend this same idea to some of the applications that I use and I found that there is a similar trend. The applications are very easy to learn and become useful in a very short span of time yet they are feature rich to make you want to come back and use them every time.
Its the same for the android and apple devices - they are so easy to use they do not even need a user manual but there is so much available on the market place/app store that there is never a boring moment as you can go there and find something new to keep you engaged.
For all the app designers out there the idea is to keep the entry barrier so low that the user feels interested to try it out but then it is important to subtly introduce features to the user that will keep the user engaged and interested.