Google catches people already looking for what you sell; Facebook interrupts people who weren't. If your service is something people search for at the moment of need — plumber, accountant, lawyer — start with Google: the intent is worth the higher click price.
Facebook/Instagram earns its keep where the purchase is visual, discretionary, or discovery-driven (renovations, gyms, restaurants, events), and for staying visible to past visitors cheaply through retargeting.
Our default sequencing for a limited budget: prove Google search first (intent converts fastest and teaches you your real cost per lead), add retargeting second, prospect on social third. Running both channels thinly usually proves neither.