Building a Social Network, Island by Island

by Jesse Farmer on Monday, May 11, 2009

A necessary condition for building a self-sustaining social network is density. We understand this intuitively. After all, a network of one person is hardly a "network" at all.

Metcalfe's Law, which states that the value of a network grows in proportion to the square of the number of users of that network, express this idea formally. The value of a social network rests in its ability to foster communication, in its connections.

If you're building a social network, whether it's a destination website or an application that exists on another social network, density must figure into key strategic decisions. Let's see how.


One way to think about social networks is as a network of networks. On Facebook, for example, if I graph the connections between all my friends I see distinct groups: my high school friends, my college friends, my current circle of friends, and my professional network.

Each group of people is more or less isolated from each other. Density exists within each of these islands, but not between them. I'm fairly certain this is a topological property of any social network.

Case Study: Facebook

Facebook started at Harvard and was initially college-only. Their growth strategy was explicit from day one: move from school to school as demand warranted. I've been told that Sean Parker wouldn't consider opening up access to a new school until at least several dozen students from that school requested an account.

Each school was an island. Once Facebook saturated a specific set of colleges it moved onto the next round. Eventually there was enough anticipatory buzz that they could launch at large, state schools without risk of fading away.

They still pursue this strategy today. After establishing a critical density among colleges they opened up access to high schools and then to everyone with an email address. From there they started moving country-to-country.

The countries where Facebook is having the most difficulty gaining traction are the ones with already-established social networks, like Germany with StudiVZ. In fact, if you look at this old map of the most popular social network in each country, you get an idea of how isolated this country-by-country growth really is.

Other Networks

Facebook isn't the only example of a social network who grew this way. hi5 has a similar story, starting with smaller markets overseas and spreading from country-to-country. Or Craigslist, by starting small in San Francisco and eventually becoming a presence in most major US cities.

The MySpace team had a background in direct marketing, which is all about targeting specific offers at the people who are most likely to respond. They started with the club scene in LA and grew from there.

The key to all these strategies was density.

If you're launching a new social service, even if your end goal is to have everyone and their mother using it, it's important to understand the impact density has on the growth

Multiple Dimensions of Density

So far the only kind of density we've talked about is network density, i.e., multiple people connected through their shared use of a service. You could call this "product density."

Sometimes product density isn't enough. Take IM, for example, or any network that requires synchronous communication. Not only do two people have to be using the same product but they have to be using it at the same time. What good is your friend being on IM if you're never awake at the same time?

Xfire, an IM client for gamers, is an example of a product that innovated in this space by tackling a segment of customers who were already interacting synchronously.

Mobile social networks take this to an even greater extreme. To connect with people on Loopt or Google Latitude not only do we have to be using the same product at the same time, but we have to be in the same place!

This isn't to say building these networks is impossible. Rather, they come with an extra handicap in the form of reduced density. Overcoming that problem has to be a key part of the product strategy.

Conclusion and Counterexamples?

Most product strategy discussions, in my experience, are focused on acquisition or other topline metrics that go "up and to the right." Instead, if you're building social software, I believe density is a necessary condition for long-term success and needs to be a part of the strategy discussion from day one.

First, understand the density requirements for your product. Do customers need to sign up for the same service? Do they need to be using it at the same time? Do they need to be in the same place? Is there anything you can do to lower the density requirement?

Second, build a "depth first" strategy. Are there any naturally dense customer segments that might fit your product? Do you have the ability to target specific demographics or segments for acquisition? Which ones respond positively and is it possible to build density there?

Once you've achieved sufficient density on one island hop to the next and repeat.

And if anyone out there can think of any counterexamples — social networks or services that got big "all at once" — leave a comment and let me know! I honestly can't think of any.