A goal of many new Web “2.0” ventures is to build a large or at least persistent community. Success is difficult to measure, but breaking into the top 100,000 sites by traffic measured by Alexa is one goal. It might be a good sign if the site designers send emails to a few people asking them to take a look at the site and after only two days over 3000 people sign up to beta test. You could do worse than to have a list of 60,000 people desperate to join your site before it leaves beta — so desperate that the site admins put a “waiting list checker” page up just so that an impatient person can see how many people are in line to get accounts before he or she does.
The site that’s done this is Ravelry. A knitter and her bored Web monkey husband put up a site intending to collect knitters’ descriptions of their ongoing or finished projects. The core idea was that knitters are always thinking of what project they should do and with what yarn, and they look at what other knitters have done as “research”. Without Ravelry, I used to google for yarn names and painstakingly trawl through search hits (and misses) and get tantalizing ideas for what to do with some particular yarn, but with incomplete information about where the pattern came from or what size needles were used. I knew that somewhere out there were dozens more projects, photos online and everything, but the knitter didn’t bother mentioning the yarn name in association with the photo so I couldn’t find it.
In theory, somebody could have predicted Ravelry could be a hit. The founders of Ravelry initially thought it could have 10,000 users, because there was clearly a community of knitting bloggers that habitually commented on each other’s blogs and even arranged meetups with people never met in person before (at sheep festivals, book signings, or local pubs and cafes). These are the people who — when a few popular bloggers raved about a yarn called “Socks that Rock” — made that yarn so popular that a bank shut down payments for the providers of that yarn, thinking that an ecommerce venture so small and so quickly popular must actually be using stolen credit cards or otherwise scamming its customers.
There’s probably a way to find such groups emerging — find people linking to each other back and forth frequently, for example, track reciprocal comments. But to successfully build a site for those people, one probably needs to understand their needs first.
 I use the term “Web monkey” with the greatest respect, implying a facility with the medium that I do not have. Hat tip to Stash and Burn for their interview with Jenn and Casey, and some of the facts used.