My name is Mijael Feldman, im co-founder of BOMBAcamp (www.bombacamp.com), an early stage startup accelerator for spanish speaking market.
How many paying customers you have for your product/service.
Visits/users/suscribers/followers are worth nothing if you dont have people paying for you product.
Best validation for a startup in its first year is to habe customer paying.
Validate a problem -> validate the solution
-> test solution -> paying customers. Dont worry about getting a million customers, focus on getting the first 10, then 100 then 1000.
Hope it helps! If you want more details of how doing this process i am open to jave a call with you.