Building a community site is just the beginning. Once it's up and running, how do you measure your success? You could take the easy way out and just go by a simple measure like number of registered members, but that's the wrong way to go. What you really need to know are things like: How engaged are my members? Are they getting more involved with the site over time, or less? Are they attracted to the web features that I spent the most time (money) on, or are they instead lining up to use the features that come standard in my CMS?
In short, you need a strategy for measuring your community, and that measurement needs to have historical dimensions so you can track trends. My manner of approaching this task has changed over time; this is where I currently stand.
1) Bounce rate. This is the measure of visitors who come to your site, take one look at it, go, "Bleah!" and leave. Ideally this should be the number of visitors that leave within 10 seconds of hitting your site. Depending on your analytics package, this figure may or may not be available. On my current post we're using Omniture, where the best I can do is measure the number of visits that take less than one minute. Given that the sites I work with are pretty content-intensive, a visit of less than a minute is a pretty definitive bounce.
A variant bounce-rate measure would be single-page visits, though you should look a little deeper here and see which page they're visiting. It may be that you have a single page on your site that meets the needs of a number of your returning visitors, in which case it's not a classic bounce. On the other hand, if this single page is your home page, it's a good bet that you're failing to engage with your content, design, or information architecture.
2) Forum posts. Not a sexy number, since this is your members doing the community thing on their own, without engaging with any of your fabulous (and possibly expensive) original content. But it's real community juice, and you want to know if posts and threads are going up or going down over time.
3) Time spent per visit / Pages per visit. These are two sides of the same coin (so it should be perplexing if one is trending up while the other is trending down). Of the two, I tend to prefer the latter, because while pages per visit is skewed by search engine spiders that crawl every page on your site, my personal bias is that time per visit is skewed more by visitors who load your site then walk away, leaving the browser open without exiting the site. I have no data to prove that, mind you, it's just my lurking suspicion.
Either way, you want these measures to rise over time. More time and more pages per visit mean that what you're building is keeping people around.
4) Visit number. I'm fond of this metric. It shows you how many of your visitors are new to the site vs. returning. Community sites should draw in more and more repeat visitors over time. Of course, this is easily skewed as well; anyone who clears their cookies will be registered as a first-time visitor the next time they visit. That's why you shouldn't look at absolute numbers, rather look on whether the numbers are trending up or down. Down is bad.
5) Success events. This metric isn't special to community sites; pretty much every serious site should come complete with an idea of what you want your visitors to do when they visit. When they do those things, it's a success event. Define them, measure them, graph them over time. Trending up is good.
And last, and somewhat least, you've got:
60 Competitive stats. Odds are that you have competitors: sites that make you grind your teeth, those evil bastards who are stealing your visitors and buying Google adwords that rightfully belong to you. Compete.com and a few other sites will graph traffic to any URL you enter, though their data certainly isn't of uniform quality. So come up with your two or three top competitors and see how they measure up against your site! You'll never know how trustworthy the data is, but it certainly makes for interesting reading.
Site analytics is where, in my experience, most community sites fall flat. All the energy goes into features; after they're live, no one really wants to follow through and see what is performing well. Maybe we're all just afraid that we're secretly failing, and pulling down those numbers will expose us for the frauds that we are. But if you don't have those numbers you never have the chance to learn from your successes and failures. Gather the numbers, report them monthly, and get better at what you do.