I wrote a couple of pieces last year encouraging social networks to open up their networks and to let partners develop modules and applications around them. Facebook, with the new Facebook platform, is the first company that I’ve seen that comes close to that vision. The implementation is different from what I outlined, but the goal is the same: let people connect with each other using a wider range of tools than one company can develop.

Just as free open source tools and commodity hardware dramatically reduced the costs of launching a startup, Facebook reduces the costs of getting adoption by giving entrepreneurs easy access to a community of more than 25 million people.

Most social networking tools face two big barriers to adoption:

• Getting users to sign up. Another registration process. Another user name. Another password.
• Building out networks. Every time I sign up for a social networking site, I’ve got to go find out who among my buddies is already there. I’m still discovering old friends who have been using flickr for a while. Given that the value in social networking sites is primarily driven by who is there, if this process is too difficult (which it usually is), I’ll leave.

Facebook’s open platform breaks down both of these barriers.

Facebook applications are inherently viral. Instead of the annoying invite all your friends links that spams all your friends with heavy-handed “Sign up for Geekstr!” email, your friends see in your feed that you’ve added an application.

One of the areas I’ve worked on extensively over the last few years is using social networks to help you filter content. The premise is that there’s a reason your friends with people — you have common interests, you work together, you live together, etc. There’s a higher likelihood that you’ll be interested in something if your friends are interested in it. (And you’re more likely to trust their opinions than those of random strangers.)

This works for Facebook applications. I added the “Cities I’ve Visited” application after a few of my friends did. Then I saw a few of my other friends add it.

That’s cheap, powerful viral marketing.

Update: Marc Andreessen has a great technical analysis of the Facebook platform. Jason Kottke has a more critical take.