From what I have witnessed, I think that signature campaign is more effective because most of people from Bitcointalk have decent knowledge about Bitcoin. In social media, it is very hard to find someone who knows about cryptos so that it is kinda useless and waste of money. If I am a marketing manager, I will never ever make a social campaign about bitcoin-relate services.
This is true only if you're advertising a business that deals exclusively with Bitcoin. In such case a signature campaign will probably find more target audience on a Bitcoin forum, like this one.
If the business accepts both Bitcoin and fiat payments, a social media campaign will find more target audience, because it addresses both Bitcoin and fiat users at the same time.