One of my favorite marketing books is Robert Cialdini's Influence: The Psychology of Persuasion. Influence explores the psychological principles behind successful sales and marketing tactics, which Cialdini defines as follows:

Reciprocation
Commitment and Consistency
Social Proof
Authority
Liking
Scarcity

He originally wrote Influence as a combination of pop psychology and consumer advocacy - each chapter ends with a couple of "how-to's" for avoiding the clutches of crafty marketers and con men. But then he apparently realized how much more money marketers would pay him for advice than consumers, so he's since started his own consultancy for the former. (I'm just joking about that. I'm sure Cialdini is a nice enough guy, but it's funny to notice the shift.)

The social proof tactic is particularly relevant to social media. In a nutshell, social proof is, when considering a purchase, the reassurance you feel to see that others have made the same decision as you. This is the reason testimonials are so common in advertising. It's also the reason your prospects ask you who else is using your product.

This dynamic obviously plays out in social networks. If you're thinking about joing a Facebook group, would you prefer the one with 4 members or 400? When subscribing to the TechCrunch Feedburner feed, you'll impressed to know that it has over 500,000 subscribers.

But what's even more persuasive is when your friends provide the social proof, also known as word of mouth, which is every marketer's dream. The closest facsimile on Facebook is the pivot on your friends list. For example, when browsing groups and applications, Facebook tells you how many of your friends are members/users.

So the other day I was surprised and impressed to see how Eric Norlin, impressario of the Defrag conference, is leveraging this phenomenon. Last week he announced the Defrag Connector, which uses the Facebook API to show you which of your friends are going to Defrag, which are still thinking about, and which have not responded yet.

I've attached a couple of screen shots so you can see what it looks like in my network. How could something like this be used in your world?

(In truth, the most amazing thing about this is Eric didn't even commision it. The developer of the app took his own initiative and pinged Eric after the fact. In other words, he's an Enthusiast (rim shot) who is using available tools to promote Eric's brand. Stay tuned for a podcast with Eric in which we ask him how the heck he's able to inspire that kind of word of mouth.)