All services based on user generated content, in their beginnings suffer the "critical mass" problem. The problem is always the same:
The answers.onstartups.com also was a potential subject to this problem. But when you have such a great number of motivated readers as onstartups.com has, the problem is almost self solving.
The simple solution is to populate the service with content from some other source - but this is not always possible (take Foursquare.com as an example).
Of course anyone can say: "just make the great service and people will follow" - but that is false vision of people who probably never went through a process of building the community.
So the question is: Do you know any case studies of good solution to the "critical mass" problem that would give some insight for people who are struggling with it?
One of the standard solutions to this is to get "fake users", that is, friends and family who will do some of the initial contributions. As well, by running a trial-beta, you can use the content generated there to populate the production version.
While it is a circular dependency here, remember that both publishers and content-producers are aware of the problem. So if you launched yesterday, no one really expects you to have a huge amount of content, but they do expect to see the potential. They also want to see sustained growth - the initial users don't really count, because they are probably a high percentage of friends and family. It's the next level of users, the first that are attracted by word of mouth and the product itself that really matter.
Of course, being able to draw in users from another location, for example, running a blog and then getting the readers of your blog to use your product, can always make things easier. Often, you may want to do this (write a blog or an online magazine with no direct compensation) to build a user-base that you can use in your next generation production.
Well I have a couple of ideas but not much time right now. Obviously this all depends on what kind of service you want to do.
But here is my basic advice. I have used this myself to great success and been involved in building a number of communities throughout the years.
Make sure that you start as simple as possible but not simpler.
What often happens is that companies build a platform with all sorts of bells and whistles and multiple channels.
Thinking they are catering for the different needs what they have done instead is creating a platform that assumes critical mass.
But the trick with communities is that you want people to hook up not to explore options.
So instead I normally use the following principles.
Build one channel with one option. Expand channels when the traffic starts to become too noisy Expand options as the user base grows That way you have lowered the barrier for what constitutes critical mass and users are much more likely to feel like they are part of a community thus stay.
I'm also running a question & answer community similar to this one, but about personal finance. Here are some of my thoughts on the subject of gathering the critical mass you speak of:
First, I agree with Elie 100% on the 'fake' users and the trial beta concepts:
But having some good initial content to bootstrap isn't sufficient by itself. First, your content ought to be indexed by the major search engines (at least Google.) Second, people arriving via search need to find what they're looking for when they get to your site. i.e. the content needs to be useful : help somebody solve a problem, answers a question, etc. If your content is useful, you've got better odds of converting a search visitor into a user that'll come again. If somebody just finds matching keywords at your site, i.e. lots of words yet little substance, then they are likely to bounce.
What's also been working out for me is participating at similar sites – as I do here. While my username is obviously an ad of sorts, I do try to post answers and comments that genuinely add value to a conversation. I think this has resulted in substantial visits to my site. (But the competition is catching on to this strategy!)
Note that I didn't have a popular blog or other site of my own that I could leverage directly, such as Dharmesh has been able to do here. I know now, though, that laying such groundwork well in advance of a venture would have been useful! Still, I don't believe that having a successful blog to leverage is a guarantee of success ... it would just have been a big help. I believe strategic thinking and hard work can more than make up for it.