With 750m users and growing, Facebook is one of the most attractive platforms for brand advertisers to reach consumers.

But there’s a problem: while it may be capturing more and more dollars from major brand advertisers, the efficacy of advertising on Facebook is still questionable.

If Facebook isn’t able to change that, it will obviously face challenges long-term.

In an effort to create ad offerings that satisfy its advertisers, the world’s largest social network turned to some of the agencies it works with. “What do you want?” the company asked.

Out of 100 ideas submitted by the agencies for ad formats that would take advantage of Facebook’s characteristics, Facebook selected one proposed by Leo Burnett. The ad format: a “Comment” ad. Fast Company’s E.B. Boyd explains:

The new unit is called the “Comment” ad. It will appear in the right-hand column, like other ads. But it will be formatted like a conversation (or, as Facebook calls them, “a story”). The brand will make a statement or pose a question, and below that will be a comment box where the user can enter a response.

If the user does enter a response, the conversation then appears in the user’s News Feed, where the brand starts to reap “earned” impressions among that user’s friends. And if some of the user’s friends comment themselves, those comments can be turned into Sponsored Stories.

As Boyd suggests, “it’s the ad that keeps on giving” — at least in theory.

Needless to say, Facebook is wise to take advantage of its enviable position to work with major agencies on new ad formats. After all, it’s easy to sell an ad unit that advertisers themselves created.

But will it work?

Developing an ad format that sells is not the same thing as developing an ad format that works. Brand advertisers love advertising; consumers generally don’t.

While the Comment ad format may seem like a great way to create brand-consumer ‘conversations‘, it assumes that consumers want to talk about ads.

In some cases, of course, they do. Certain ads organically produce a lot of buzz, both good and bad. But therein lies the problem: many of the advertising campaigns that go ‘viral‘ don’t necessarily do the best job at selling a brand and its products and services. In many cases, beyond a short-term spike in visibility, there’s little measurable impact.

Yet increasingly, advertisers are shooting for that short-term spike in visibility, and the best way brands to create catchy (and sometimes wacky) ad campaigns that aren’t really aligned with their brand’s positioning in the market.

Conceivably, ad formats like the one Leo Burnett has created for Facebook will only encourage brand advertisers to focus more on creating content than creating effective branding, which it’s easy to forget are not the same thing.

After all, if you expect Facebook users, who are more likely to click on ads with a picture containing beer, to engage with the Comment ads, you’re probably going to need to focus on content more than anything else. And that may not be such a good thing for brands not in the content business.