Every single product should try to optimize for the users' LTV (life time value).

Easy to say, hard to measure. But the closer you get with the measurement, the less noise there is, and the harder it is to game the metric.

- clicks: lot of noise, loose correlation with LTV

- zaps: less noise, higher correlation with LTV

So the question always is - how to define the LTV for the user of the product in a measurable way. Simple concept, hard execution.

I would probably go with some composite metric for engagement. Daily, weekly, and monthly unique engaged users.

Extra tricky thing here is that each user has different things in their LTV, and mostly they are unable to express it even if you ask them.