
Lesson from Social Studies
In social studies, we learned about something called The Tragedy of the Commons. Say you have an unowned commonly shared piece of land in which multiple sheep grazers border. Each individual sheepherder is incentivized to overgraze their sheep on the land even though in the long run the land is ruined by everyone’s misuse.

The problem with the commons is that no one has ownership over it and so the common land is abused. The tragedy part is that in the long run, this behavior is actually harmful to all parties involved.
Internet Commons
On the internet we have common resources as well that get abused, but in particular we have communities that devolve. Probably you have come across the occasional message board in your searches that are littered with spam messages and SEO keywords. You’ve also seen how Yahoo! Answers has devolved into a playground for ridiculous answers.
This is the tragedy of the commons in action.
Let’s look at two solutions:
Curation
I remember listening to an episode of the Stack Exchange podcast with @spolsky and @codinghorror. In it, they detailed just how meticulously and stringently they curated questions and even whole communities on their Q&A site Stack Overflow. If you’ve ever used stackoverflow.com, you know that it is probably the single most important site for programming answers across all topics on the web.
Spolsky and Atwood did not luck into this. They knew very well, the problem of the commons. The quality and reputation of the answers on the site are in direct correlation with the amount of community policing and maintenance put into the site’s “commons”.
The cost of curation
In our example from social studies, Stack Overflow supplies regulations and maintenance staff (like a government) to ensure the commons are not overgrazed. This is a non-trivial cost: in time, in thought and in money. Whenever a client tells me they want to incorporate “community” into their site, I let them know right away that it’s not just about putting up a bulletin board and a link to Twitter. On the Internet, every bulletin board and every forum is a stretch of commons, and curation of the commons costs real money. It can’t be an afterthought.
Another model
There is another model for community however that doesn’t involve outside curation.
When I wrote my blog post about The Pruitt-Igoe Myth, I was fascinated by the concept of Defensible Spaces, by architect Oscar Newman.

To recap, the Pruitt-Igoe housing complex degenerated into a den of scum and villainy partly because of architectural concerns. Common gallery hallways, “skip” elevator stops and centralized communal space per floors were lauded by the architects of the day as community building features but ultimate suffered from the tragedy of the commons.
Since it was a low-income housing project in a town that was suffering economic decline - curation of the project (in the form of government funding for maintenance) was not enough to overcome the tragedy of the commons. With no curation, the housing project fell into disrepair and was ultimately completely demolished by 1976.
With the concept of defensible space, Newman points out that in the Pruitt-Igoe complex, the majority of tenants lived in long commonly shared gallery hallways. These common spaces soon deteriorated due to trash, vandalism and graffiti.
Planning:

Reality:

However, he notes, where there was a short landing shared by only two families, that common space was well-kept, maintained and flourished.
The idea of defensible space is that the more a person feels ownership over a piece of land, the more likely the tragedy of the commons will not occur. Consider a city street with a series of single family homes on it. Each household on the street considers the sidewalk and the street in front of their house “semi-public” - they have a stake in that street and maintain their sidewalk and watch over any suspicious activity in front of their home.

Now consider a six-story walkup (20 -30 apt units) in that same place. Here, the sidewalk is completely a common area and the street in front is owned by no one. In fact, even the steps leading up to the building are considered “semi-public”. Because of the lack of felt ownership by tenants of the building, the sidewalk and street start to suffer from the tragedy of the commons. Trash piles up, graffiti is unwashed, no one is looking out for suspicious characters.

In the first case, we have defensible space. Ownership is exerted upon what are ostensibly common areas, owners clean the trash, paint over graffiti and repair small issues. In the second, we require curation - we must hire a super to look after the sidewalk or government services are needed to clean the street and maintain it.
Defensible Spaces on the Internet
On the Internet we have examples of defensible spaces are well. These are communities that are self-curated through semi-public spaces.
Twitter for example, is a huge public community that is opt-in. You choose who to follow and see anything they post. Others can follow you but you won’t see what they write unless you have also followed them. In Twitter, you wall off your own defensible space. You decide who is in your network and who is out. So even though almost everything is a huge public commons, your have your own space that you can defend.
The exception to this is @mentions and Direct Messages which can only be defended in retrospect. Meaning that anyone can mention you and intrude into your space. Only after the intrusion can you block them and keep them out.
Facebook also is an example of defensible space. You choose to link to your friends and they choose to link to you. This allows you to self-curate your social network and “own it”.
However, there have been times where your space becomes “public” due to changes in privacy settings or the “friend of a friend” sharing problem.
TL;DR
In short, the tragedy of the commons is relevant to the Internet where there are long stretches of “common” space. To deal with this problem there are two solutions, the first is the use of curation to maintain control and quality over a community’s interaction. The second is the create defensible spaces within the commons for users, so that they can exert ownership over their own virtual plot of the Internet.
Thought comments flames? @marksweep