A social proof strategy is a requirement to have from day one of freelancing. As I personally see it, there's nothing more valuable than experience which the prospective employer can cross-check with the previous employer. That's the image the markets gives to me, which is hard, and kind of depressing for those who are total novices.
I hope that hard work as an independent contractor, be it content writer, digital marketer, or whatever, if it resonates with the customers, and one is tracking it, it's going to show numbers. That's another cold-stone truth like verifiable experience. And I don't mean just testimonials from the few of one's clients that kindly gave feedback and permission to publish it.
I mean, if a struggling freelancer wasn't able to get any kind of social proof involving a third party, still there aren't excuses to not create it oneself. That's why I say it a content strategy and a way to track it must be implemented from day one, with every social and asset touchpoints having a means for tracking and extracting statistics.
This isn't hard, but also isn't easy. Solid work hours have to be put into achieving it. The best advice I can give to get something fast is a two step approach:
(you need your own portfolio and/or blog site CMS)
1) Join as many social networks as you think you could use to work. Also freelancer platforms, content sites, and similar communities, and interact with the community often
2) Experiment through executing a content strategy in each, or an integrated one that includes as many as possible of them. Something that directs interested persons to your site
Once you shared content (owned, earned and third party) for three months or more in each, you'll have a clear picture which sites are a source of traffic for the your site. If you continue delivering value and if you create a critical mass of interactions and engagements, that is going to create traffic that will show up in the monthly reports of the solution you used to track and extract statistics.
These are just my two cents of advice. There are many more ways to create social proof without selling anything to or interacting with your potential clients.
I leave you with another article, a great way to start thinking how to go about designing your own social proof strategy: 12 Jaw-Dropping Social Proof Examples To Grow Your Business
This is unrelated, but the ultimate takeaway from that article, for me was that it’s actually a very neat example of how to make a rich content article.