Micro-SaaS

I've been reading a lot about micro-startups lately, and one of the primary questions I am failing to understand is that how are entrepreneurs able to acquire customers for their product?

I am skeptical of the capabilities of the micro-saas culture, the only way it seems to work is if there already a customer base for the product and somehow you are able to capture the customer base by either shorting the competition or building tremendous value.

The customer base itself is developed through twitter and other social media channels. I don't think these micro-startups have complexity or challenges that native startups inherently have (product-market fit)

Any good experts or resources that guide me through this turmoil? What I am expecting is a step by step guide that can help me generate let's say $500 MRR?

I understand what I am requesting is difficult, however I am only looking for the way?