When a user gets a like, a retweet, an emoticon notification, the brain receives a flood of dopamine and sends it along reward pathways.
Even apps like Goodreads and especially language learning apps (I am talking to you Duo lingo) rely on this Dopamine rush to bolster participation, of course driving up advertising rates.
Of course there is some rhyme to reason. Setting concrete goals has been proven effective. I want to lose 12 pounds by May 31st. However some days I want to learn a language on my terms.
The Side Effects of Endorphins 