I recommend that anybody who is interested in this read section 9 in this paper: "NOTIFICATIONS AT FACEBOOK". It brings into focus real ways that this technology is used and is useful.
So cryptic and hand wavy description in paper but hey I'm glad they have good results. Going from pedals to combustion engines clearly has efficiency benefits but they gloss over the real flesh of the fruit when they don't describe how their particular engine is better or tuned. Wish they had more talk of notifications as Markov chains and how they evaluate interaction