Targeted Google ads (for the type of product we were building) pointing to a landing page where we collected email addresses.
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Most new founders think that blasting your 'startup' to Product Hunt, Hacker News, Indie Hackers, Reddit, Twitter, etc. will result in first customers -- that that's 'marketing' -- but that's far from the truth for the majority of products. And contrary to popular belief, the chances your product is one of the exceptions is near-zero.
Those social media platforms bring in 'tire-kickers' and devs that value their time at $0, not customers. These aren't the first users that you should be listening to, because they will always complain about price, lack of niche functionality, etc., yet it's pointless to listen to them -- because they aren't buyers.
You want to market towards buyers, not just users, and ads are a good way to do that for early companies that have no brand awareness or distribution.
Unfortunately not everyone has resources to start with ads (especially bootstrapped firms or solopreneurs), and in some way growing organically and slowly gives you more time to develop product better.
Having said that, I am in agreement with the essence of this reply.
My company is bootstrapped, and I'm a solo founder. If you don't have any money to grow your business, then you aren't going to be able to do much until you have some money. Hard truth, but it takes money to earn money -- either your money, or somebody else's. You have to get buyer's eyes on you, somehow.
You can start with a small budget and build from there. For the first year I only spent £100 per month on Adwords.
The bigger problem is Google et al make it very easy to waste money on their ads. You have to pay a lot of attention to the detail and constantly fight against their defaults which are nearly always in their favour rather than yours.
That's me in 2018. Spent a good amount of money in Google Ads and only attracted users that we did not want.
Just putting negative keywords wasn't sufficient, and even one miss was enough to waste all efforts.
Best way is learning through practical experience by starting with small budget, and increasing slowly with refinements.