Common features of habit forming apps?

What features make an app habit-forming? What are some examples of apps that have done a great job executing on this?... Except for the obvious giants like Facebook.

I wrote an answer addressing this here: How to make a product sticky? Like Facebook or Instagram.

Ask yourself this: Is your app a vitamin or painkiller?
Painkillers have an obvious need, quantifiable market and can be monetized. Vitamins are more about emotional need, not efficacy.
Which do you think are Facebook and Instagram?
Painkillers!
Habits form when not doing something causes pain.
For every aspect of your business, you need to move from pleasure seeking benefits to pain alleviating benefits.

Read the full answer here