How to attract users to a new startup?

Question

Internet advertising

I'm sure there are many interesting startups that die early because they simply haven't found their critical mass.

From nuclear physics:
Critical mass is the minimum amount of fissile material required to start a self-sustaining fission chain reaction.
What is fissile material? What is a chain reaction? And how does this apply to our question?
If we adapt this term for our question, we get:
Critical mass is the minimum number of users required for the product to become useful and the viral loop to become active.

Let's discuss, what is the way you shoot a startup? As did Instagram, Airbnb, Pinterest, WhatsApp and many others.

Answer 1

Conventional marketing, there are more than enough books, it also requires a lot of money.

Answer 2

Give examples of an interesting startup that has died.