This is of great concern to know what is the rate of conversion due to online promotion/ social media promotion. Since there is no tool as such to calculate the exact number of footfall due to online promotion/ social media promotion.
But then you still can have the notion of if online marketing is working for your business or not.
You are expected to have the slightest idea of what an average number of customers visit your restaurant on a daily/ weekly/ monthly basis. And then you can compare the increased rate of customers coming down to your restaurant.
Not that you will get an exact number of people coming due to online promotion but you can have an average idea, whether or not to invest in social media marketing for your business.