Even with the trend for ad micro-targeting, the success of advertising campaigns still depends largely on their reach and scale. "If not enough people participate in social ads, then it's questionable whether it's worth bothering," Frank said.

Moreover, it's also not necessarily true that social ads by definition pack more authenticity because they carry the endorsement of friends, he said. "I'm not sure it's so easy for everyone to buy that, even with all the disclosures and transparency."

This point gets cloudier with the question of whether people should be rewarded in some way for agreeing to participate in social ads by the site publishers or marketers, Frank said. The intrinsic value of someone's endorsement of the product is that their motivation is supporting a brand they like without having to be incentivized to do so with compensation.

"There's a fine line between expressing your views and being part of a marketing campaign in which you expect to be compensated," Frank said. "That's one of the big questions: How does compensation enter into the world of social media without corrupting the conversation?"