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Facebook is a business, after all. Research firm eMarketer recently estimated the company will take in more than $4.2 billion worldwide in 2011. The bulk of that--$3.8 billion--is advertising revenue, and it's no secret that smaller businesses collectively comprise the big sales whale. That's true, too, for one of Facebook's chief competitors: Google pursues a similar target with its AdWords platform, which likewise continues to evolve.

So is one a better fit than the other for small businesses? "They're very similar but different advertising paradigms," said Larry Kim, CTO of online marketing firm WordStream, in an interview.

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If you're comparison shopping AdWords and Facebook Ads, these guidelines can help you make the right decision.

The Disclaimers

There's absolutely no rule that says you can't advertise on both sites--or neither, for that matter. Nor are they the only games in town. There are indeed other options, including Microsoft's Bing-Yahoo platform and industry-specific channels. But both AdWords and Facebook Ads warrant at least a look simply by virtue of audience size. Google still dominates the search game with nearly two-thirds market share, according to Experian HitWise. Facebook, meanwhile, is The Social Network--it seems just a matter of time before they announce 1 billion accounts.

How They're Similar

Both Google's and Facebook's advertising platforms share some fundamental things in common--that massive potential audience, for one. Kim, who toils daily in the world of clicks and conversions, points out that both are primarily pay-per-click (PPC) advertising channels, even though both offer other advertising options. Likewise, the basic systems for each are largely self-service for smaller advertisers. Kim also points out that both offer free marketing tools, such as Facebook Pages and Google Places, that don't require an ad buy. Another similarity--the opportunity to run campaigns targeting very specific market segments--ultimately leads into the differences between the two platforms.

Advertising Intent

The fundamental question to ask when comparing Google AdWords and Facebook Ads: What is your goal for the campaign? Your answer will go a long way to determining which channel is a better fit. The reason is fairly straightforward: the Web audience doesn't search Google and check their Facebook Profile--excuse me, Timeline--in the same way.

"SMBs really have to figure out what they are trying to sell and to who," Kim said. "Is it branding, or are you trying to sell something very specific?"

Kim said that because a Facebook session tends to last much longer than the typical Google search, the former be a better match for building brand awareness or getting a specific message across. That's even more true if that message is intended for a very specific audience, such as a university alumni group or people that like True Blood. Such specific, people-oriented messages would be more difficult to do with AdWords, Kim said, though not impossible.

But Kim notes that Facebook isn't strong in pure Web searches; Google has the clear advantage there, and as a result could be the better fit for driving actual clicks and conversions around specific products. It comes down to intent: Whereas Facebook users might just be checking up on their friends or posting vacation photos, Google searchers typically have a much more specific goal.

"If you're trying to sell wireless headphones or something like this, it would be very difficult to target that level of granular intent on Facebook, whereas you could just buy the keyword on Google," Kim said.