1> Early adopters come in 2> Because they are early adopters, they tend to know about lots of cool things 3> The site becomes popular 4> Late adopters join

1> Early adopters come in 2> Because they are early adopters, they tend to know about lots of cool things 3> The site becomes popular 4> Late adopters join 5> Site gets diluted, irritating 6> Early adopters move on 7> Site is a large, popular wasteland I'd point to Metafilter as a site with unusual longevity due to a closed subscription process. They recently covered the Hexayurt Project, however, and I was not impressed - a bunch of braying jackals who hadn't RTFA screaming their ignorance in public. So even Mefi is on the decline, apparently, at least in the comments. The front page is still pretty decent. But that, really, is the key: you need to stop adding posters when the site is cool and hip, to draw out the period in which the site is providing early-adopter content. Moderation systems won't do it: they get swamped by bad moderators over time, witness slashdot's meta-moderation corrective. Good luck.