I think OP is talking about paying to place an ad for your comic -- like putting your comic in the ads on TWC or buying adspace through Project Wonderful!
When I asked my readers where they'd found my comic, a decent number of them said they'd found me through an ad, and the only paid advertising I'd ever used was advertising on fellow comics' pages through Project Wonderful -- so while I never saw an EXPLOSION OF READERSHIP or anything from them, it seems like they definitely worked to reach a some dedicated readers!
It's been several years since I paid for anything, though, so I can't speak to anything other than: NEVER pay for ads on Facebook.