There are a few things that stand out to me from your message that I would like to address. First of all, it seems to me that you have the opinion that (most) new customers gotten from social media are bad. I am wondering why you think that is. The same people that are on social media may Google for a car repair shop, come across your website, and become a customer that way.
Secondly, advertising on social media can cost you as much as your budget allows in other words, you determine how much it cost. The big question is, how much do you need to spend in order to see the results you want? This comes down to testing what works and what doesn't work. Instead of trying yourself, you could outsource social media advertising and let an expert do the work. This would require additional cost but will deliver you better results and probably saves you money compared to trying it yourself. Also, when you outsource it you have experts dealing with changing trends as you refer to as "The shop owner knows that what worked last week might not work today but might work fantastically next week".
The beauty of social media advertising is that you can advertise to the customer you would like to have. Since you are able to provide information regarding people you would like to advertise to. There is also much more to it than just getting likes and views. Instead, you call for an action in your ad, for example, you could ask the viewer to fill in a form with their name, phone number, and email address. If a viewer fills out that form it means he is interested + you have their contact information enabling you to reach out.
Lastly, I would like to mention that social media advertising allows you to closely monitor a variety of metrics and in fact is the only advertising method that shows your ROI as opposed to the advertising methods you mention: advertising via billboards or matchbooks. It is true that your ad will be shown to people not owning a car (but isn't that the case with most type of advertising?), but that takes time for the AI algorithm to optimize for that.
100% agreed. You can track numbers with social media advertising...
