The sad thing is, a lot of these apps are really successful in the outset, but then end up burned as the companies try, heavy handedly, to monetise.
As much as the startup world sees themselves as morally above the like of Google who will sell out customers, many startups seem to sell out both their customers and their product, making fundamentally incompatible changes with what they were built on in the pursuit of cash.
There are many dimensions in which to monetise, the simplest being getting users hooked and then charging a fee, and others that are more complex, and all of these vary in how much money they will bring in and how many users will reject them.
What I don't understand is why startups so often choose monetisation strategies that have such huge drawbacks in the context of their "killer features". In this case, in order to sell out their users (and build profiles ripe for exploitation in the most honest, business sense), they also had to sell out the purpose of the app: anonymity.
It's likely that in an alternate universe, YikYak tried something with a lower yield to what they saw with this strategy, but didn't headshot their customerbase.