The substance of social proof is that it shows that a lot of people have faith in your product and trust you enough to pay you to use it. It's not just a gimmick, it's a concrete psychological trigger.
Yes, of course social proof has substance as marketing strategy. But gaining trust is orthogonal to actually being reliable. (this is still about the top comment)
I disagree about trust and reliability being orthogonal. Unreliable products burn their bridges rapidly. Social proof can take many forms, from raw total user numbers to user testimonials. You're not going to collect testimonials if you have a shitty product.