How do you create a site that keeps people on your pages? By creating a site that's easy to leave.
Traditional web design often focused on keeping people on a site by reducing the number of exit points: with few or no external links, the logic goes, people will stay longer.
It doesn't work that way. The Internet is designed for hyperlinks, lateral exploration, serendipitous discovery. When you cut off exit routes, you're cutting off your site's circulation, and you're creating a stagnant site.
And people don't like to visit sites that feel cut-off from the rest of the Net. Just think of how annoying it is when you get trapped in one of those spam sites with the endless pop-ups: window after window opening until you think you'll never escape. It doesn't make you want to visit that site again, does it?
Healthy circulation — in and out — is even more important in a user-driven community. The experience of porousness, of connectedness to the larger Internet, is crucial to user engagement and participation. Think of some of the most popular Web 2.0 communities: Technorati, del.icio.us, digg, even Facebook: all of them build engagement through porousness, through pulling in the best of the larger web and letting users tag, remix and search it.
What comes in must go out, of course. All of that bookmarked, tagged, aggregated and shared content points to external web sites: people come in and out of these sites all day long.
And that principle of porousness doesn't just apply to sites that are deliberately set up as content archives. Any online community can benefit by embracing porousness: by highlighting, aggregating, republishing and remixing the best of the larger web.
Porousness can mean something as simple as adopting a tag for your site, and inviting people to tag their external blog posts with that tag so you can republish their posts. By making it easy for people to contribute to your site — without requiring them to do their blogging on your own platform — your site's content and freshness expands. You get a ton of inbound links from all those people blogging about you (hello, Google!) and you get lots of people reading about you on those external blogs.
What's in it for the bloggers? Traffic back to their own sites — from the highlights you're republishing on your blog, linking back to their original posts. Yes, you're pointing YOUR visitors to THEIR external blogs — but you're getting back many times the energy and interest from all these folks now blogging about you.
The alternative is to build a big wall to keep all those visitors locked in your own site. But any wall that keeps people in keeps even more people out.