Because you can be interacting and selling your services with no investment at all, paid ads are more lucrative. Still, you can use social media for free to leverage its power and reach the far-off audience without spending anything.
It allows you to evaluate how your strategy is working and whether it is bringing fruitful results or not. The measurement factor will enable you to work out your ROI from different campaigns. If something is not working, you can either abnegate it or put more effort into it so that it starts producing the results as expected.