I believe it is their simplicity that makes social networks like Twitter and Instagram sticky. If a service is easy to use, people are more likely to use it. The more complex it is, the less likely people are to use it.
I think this idea has a lot of play especially in the early adopter scene. It's a mere fact that "apps" are a dime a dozen, especially in the iOS ecosystem. To justify adding another icon to the home screen and the time taken to understand its function the app must be simple. For any service/app/whatever to gain "stickiness" its utility has to be apparent from the very first use. Whether that utility is realized or not will determine its initial success.
Afterwards, the app's continued success may cease to be wholly about its simplicity and more about the authority of those users it does have and the voice they lend to the application. Intial stickiness and long-term success are slightly different beasts but simplicity contributes to both.