Do paid social media ads work? Surely we can still use social media with minimal cost?

Of course we can, but many of the social media ads available to us are targeting our buyer persona (provided we know who our main buyer persona is), rather than 'shouting' to anyone and everyone in the hope some may become leads.

It's important to differentiate between each social media's platform to determine what the priority is. For instance Instagram could drive traffic to your site, which is suited to prospective customers who are in the consideration stage of the Buyer's Journey. 

Facebook's content on the other hand may be geared to those who are still becoming aware of you and your products/services, therefore might be more selective in their content choice.

In any case using each platform with minimal cost is great, but using it effectively and combining it with selected paid ads can only surely only lead to greater things.