Depending on what stage your startup or project is in, you'll have to either focus on traction or scale.
Traction - Building an initial user base. This will (and should be) done manually. Yes, that means you have to do real work. Your first 100 customers / 1000 users are your most important and should be treated like royalty. Reach out to them via personalized emails, or even handwritten letters shipped via snail mail. Traction DOES NOT scale, but it lays the necessary foundation to collect feedback and musters a core user base.
Scale - You should only worry about scale once you have successfully established traction, are experiencing steady growth, your retention rate is respectable, and you have reached "product/market fit", which basically means that at least 40% of your users explicitly say they would be very disappointed if you discontinued your product, as stated by Sean Ellis. Then, and only then, you can scale your business and employ those sexy, more advanced growth hacks.
You can use Survey.io for customer development and to see where your product stands in the mind of your customers.