1. First one I started 10 years ago. I built a bot that auto DMed people in various internet forums. My first 100 users came from that. The product is highly shareable, so it quickly grew. Now it's 1.6M users (most of them free).
2. Second started 3.5 years ago. My first 100 users came from simply emailing the newsletter list from my first company. This product has no free plan, so it became profitable instantly.
3. Third started 1 month ago. And it's been a struggle. I got 10k free users just by emailing my list, but 0 paying users. So I tried ads and had similar results from the ads. Now I'm taking a step back and understanding why they aren't paying, which involves just emailing them.
Summary: once you have an email list and viral social loops built-in, marketing gets easier.