Paying for FB ads doesn't always reach your target audience, and yes, like pw66 said, every other competitor paying for ads will hit the same people the algorithm thinks will fit, so you just made it the same as looking up "sign companies" in a phone book, and paid more for it.
Having a FB page can be good, it's free, takes minutes to post random jobs you do so people will get an idea of what you do. Your customers will usually follow you, and if you can tag them, or if they share posts you made of jobs you did for them, or they make mention of you, all of their followers see them and may check you out, which is probably as, or more effective than paying for ads. FB isn't what I would consider a major advertising platform for a sign shop because it's more of a business to business industry than selling consumer retail products that the masses buy. The nature of the industry makes it tough to get in front of the right audience, not many troll FB looking for signage, but it can still help. We get inquiries and jobs from ours that we might have never gotten if we didn't have one, have never paid for target ads, and almost no time or effort on our part.