At Gossip (https://thegossip.app) we create a landing page with a sign up form and posted on on slack groups, twitter and other social media. This way we ended up with 1500+ folks excited about our product!
Through #buildinginpublic via Twitter, word of mouth.
We personally onboarded our early users one by one so we understand them more and are able to find other people with similar interests/profile
Thinking back to my first real successful app (the worlds first dating CRM) - we ran Facebook ads. It was easy to target people who were; single, in certain cities, with specific interests. I know not everyone has marketing budgets but when you do, it can work, as opposed to posting organically on social and in communities hoping someone fits your target persona.
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