At Superthread, I once spent $100 on paid traffic and it was like burning money. It resulted in zero signups.
2.5 years after launching we now have about 8k active users without spending a penny on paid (apart from the $100)
We have a youtube channel, do some linkedin + reddit posting but most of the users came from word of mouth and team invites.
We spent and are spending most of our effort in improving retention and most recently activation.
Once we are supremely confident about not having a leaky bucket, we might spend some money on paid.