> "Then who pays for it and why?"

Many people believe social signals help increase their search engine rankings and/or conversion rates. Some of those people buy likes, shares, etc. for that reason.

> "What is the incentive to fake-like advertisers on facebook?"

Perhaps people who offer those social-seo services control thousands of robot-facebook accounts and they want their robots to seem like real, active users? It seems possible that they would automate some random activity (in this case liking virtual bagel's ad) while they're waiting around for orders.

Just guesses.