The best review is the one that is no one even asked for.
When users of your product or service create a post for you on social media, detailing how much they value your product or service, that is the best social proof you can ask for. They've gone the extra mile to recognize your value to them, and to share it with their network.
An endorsement from a happy customer in this fashion is the epitome of social proof, especially if they themselves already have an established brand.
It's why influencer-focused marketing exists, or why celebrities endorse products. In that situation, user-generated content has become monetized, so if you can get a LinkedIn post about how great your product or service is written by a user at no cost to you, you know you're doing a good job.
When writing a
cold email, always take the time to see if there is any user-generated content you can use to further increase your
conversion rate. You can quote or point to content freely, as your user has created it simply to showcase how pleased they are with your product or service. For a prospect, there is no better proof of how well you work than for a fellow customer to do the work for you.