Critical Mass isn’t a term we use in email marketing. It’s true that the larger your list the more absolute response you can expect, but results scale linearly along with list size. Your open and click rates are the same whether you have 100, 1000 or 10,000 subscribers, assuming of course that they’re all equally well-qualified. Mass helps, but it is never technically critical.

Social media – Facebook in particular – is different. There, success is a function of conversation and engagement which creates some visible social interaction around a brand. Conversely, it can be difficult to build momentum in social media without an audience of a certain size. If a brand generates a 1% interaction rate from its audience, it needs 100 fans for each response to a post. Driving a lot of comments and conversation help show that a brand is lively, conversational and approachable, but a Facebook page without interaction does not create the same inviting feel. It’s like the empty restaurant during the dinner rush. If nobody else is eating there, why should you?

Facebook’s latest version of Insights does an excellent job communicating the brand impact of this engagement. In addition to the number of fans a page has, Facebook now publishes a number for “How many are talking about this,” which reflects the total number of people who have liked or shared or commented on a brand’s content. Additionally, Facebook Insights now publish statistics on “Reach,” which is the total number of people who see the brand’s content on the brand’s page as well as in their own news feed and other places across Facebook, as their friends interact with the brand. For example, a brand that has 500 fans might have a reach of 2,000 people through viral interactions – each time a fan comments on some brand content, all that person’s friends may see that interaction. That’s the viral appeal of Facebook – that brand interactions are witnessed openly by your customers’ friends, turning comments and “likes” into tacit brand endorsements.

As reach goes up, more people are exposed to the brand, creating more fans and driving a virtuous cycle. So on Facebook there is a point where critical mass is reached – where an fan base is large enough to drive visible conversation for a brand that bounces around Facebook and brings in even more fans. The actual number, as you may have guessed, depends. The more a brand drives interaction (which is a function more of its content strategy than the brand alone, as we’ll see), the fewer fans are necessary to reach this sustained and growing fan ecosystem. Anecdotally, I see many Facebook fan pages run up to about 300 fans quickly, then stall out. At the same time, many pages that reach 500 fans quickly grow to 1,000 and beyond. So absent perfect information I’m comfortable saying that Facebook critical mass is about 500 fans.

How do you reach Facebook critical mass? Here are some suggestions:

Post Frequently: News Feeds, unlike inboxes, are transient and permeable. Everything that flows in flows right out unless something – like interaction – causes it to swirl around for a while. Posting on your company’s page will remain visible to everyone visiting the page, but most of the interaction around your brand happens on the News Feeds of your followers. The more friends they have and brands they like, the less opportunity your content has to even hit their radar. Facebook publishes impressions on each post for Facebook page administrators to see. If you have 500 fans you may see that your post has fewer than 500 impressions. That’s because other content pushed it from News Feeds before your fans got to it. As your interaction goes up, your posts linger longer, so your 500 fans may generate 800 or 1000 impressions. Here is where the viral impact starts to take hold. But to get there you have to post frequently enough so that you are increasing your chances of getting into News Feeds when your fans are online.

Aim for Interaction, not Conversion: As email marketers, we’re closers. Our messages are commonly designed to drive a purchase, conversion or other direct response. But the tactics with Facebook are different. To grow your Facebook audience, your content should aim for interaction instead of conversion. “Register for the Conference today – click here” may drive some clicks, but nobody on Facebook will see them. Instead, a post like, “We’ve just announced our 4 keynotes – which are you most looking forward to?” invites responses and interaction, which will be seen on News Feeds all across the service.

Use Interaction-friendly Content: Questions and comment invitations like the example above are one way to drive interaction. Another is to spare the 1,000 words and use a picture instead. Glossy product shots are OK, but if your aim is to drive interaction then candids from the office or the last conference may work better. People are likely to comment on pictures of themselves or people they know more than products they like or events they may attend. And because Facebook is far more casual than your website, mobile phone photos are not only acceptable – they are de rigueur. Too much polish makes a brand feel inauthentic on Facebook.

Sponsored Stories: Facebook offers a number of advertising options. My favorite is Sponsored Stories, which lets you target friends of your fans with ads telling them that their friends like your brand. Sponsored Stories amplify what Facebook does best – enables your fans to recommend your brand, explicitly and tacitly. And Sponsored Stories become more effective as your fan base grows, as the universe of people who are friends with your fans increases exponentially as well.