If you're standing in front of them with a notebook in hand, it's hard for them to avoid saying something. So that's step one.
Step two would be to interpret nothingness as (negative) feedback that your product wasn't interesting enough for them to bother commenting on. Which probably means you're not solving a deep enough problem.
This problem is really hard for whatsapp-like products, since people already have their problems solved pretty well in that domain and so they are less likely to find a new product particularly interesting. If I were advising a startup who were making a whatsapp-like product then I would tell them to find a narrower niche where they could solve a deeper problem.
On a product I worked on we emailed every user that didn't engage with the product after sign up and requested feedback on how we could improve the service, what is lacking etc.. tried many different phrases but it was about 1 in 500 that replied and gave serious feedback.
Making current users even more happy is one way to go, they are more likely to provide feedback but it skews the perspective since we don't get any feedback on what the unhappy users were unhappy with.
Now, suppose it is a Whatsapp-style product, there must be a way to gather feedback even for them?
I suspect a lot of people that never replied would be people the signed up but never got around to trying the product properly or people that immediately saw that it wasn't ever going to solve their problem.
Would be interesting to correlate the emails with how engaged the user was after signup.
Personally as a user the only time I would try to engage the creators is if the product was very close to my use case but certain aspects didn't work the way I would need. This being for B2B where a potential customer is going to be more engaged, B2C the reply rate to those emails is likely always going to be tiny.
Step two would be to interpret nothingness as (negative) feedback that your product wasn't interesting enough for them to bother commenting on. Which probably means you're not solving a deep enough problem.
This problem is really hard for whatsapp-like products, since people already have their problems solved pretty well in that domain and so they are less likely to find a new product particularly interesting. If I were advising a startup who were making a whatsapp-like product then I would tell them to find a narrower niche where they could solve a deeper problem.