I could give a concrete answer based upon the psudo-science of online community building, or an enigmatic answer that really doesn’t reveal a whole lot. Instead of either of those choices, I would rather answer this question by finding the different ways that people have found significance in this question.
Some people want to know if Social Networking is reaching a critical mass? They want to be able to use their anecdotes as evidence for why Facebook is the most popular network or why Twitter has become the default conversation platform for conferences. I believe that a Social Network reaches a critical mass if people find it useful, if it becomes a place for relationships to be more than just connections. This is easy to quantify. If your network causes you to daily learn more about the people you hold dear, it has reached a critical mass for you. That is why Diigo has reached critical mass for some. That is why Plurk has reached critical mass for others. The relationships are meaningful, it is just the technology and the ease of use that gets in the way of widespread adoption.
Some people want to know if open source has reached critical mass? They are intent on looking for a way to support the software that maintains a community of users and creators who are one in the same. They want to say that they saw the tipping point when Linux became a real alternative to Windows or when Moodle became a real alternative for Blackboard. Open source has reached a critical mass because people use it without thinking about it. Apache and mySQL are the backbone for entire worlds online, and they aren’t given a second thought. This question is relevant only in the idea that the communities required to create this critical mass are essential. An open source project reaches critical mass when the leaders of the project can leave or change and the project still continues to grow. I believe that Moodle will critical mass when Martin Dougiamas is no longer shepherding along.
Some people are much more analytical. They want to know just how you reach the numbers of to create critical mass within a community? They are interested in just how many data points are needed in order to say that a site is a success. They will give the percentages of people who will contribute to a social site, the percentage of lurkers, and the percentage of people who will sign up and never return. This question is important, but it is false to assume that any community is the same as any other community.I don’t believe that a pattern works all of the time, nor do I think that it is too important to pay attention to the people who say that a community lives and dies with numbers of posts or with the amount of traffic. I believe in the power of a single idea to spark an entire community and to sustain it. I also believe that any community can reinvent itself with every iteration. Critical mass is achieved when people believe that there is a thread that binds all of the people to the community itself. If it is a single question, and single person, or a single function: A community will thrive when people find a connection to IT and not just the people in it.
Then of course, there are people who just want to know what it takes to reach critical mass on a single tool, like what does critical mass look like on a wiki? They are attempting to weigh in on exactly when a niche wiki can sustain itself with new edits on a regular basis. They see the wiki as the community and the information as the currency within it. I happen to agree that Critical Mass on a wiki is very difficult and it is something that few wikis ever achieve. However, I take a different stance on it, I think. I believe that there are very few reasons to attempt critical mass within a wiki. Wikis are meant to gather information and chronicle evolving ideas. There are very few ideas or projects that are ever-evolving or that can sustain the attention span of any group of individuals indefinitely. I believe that critical mass is an illusion within a wiki. Wikis are born, are used when they are useful, and die when they are not. While the information may continue to be referenced, continuing to expand it or build out new branches simply does not make sense. To put it simply: When you have answered the question you were asking, you do not continue to ask it.
So, there you have it. Critical Mass in a community is individualistic, improbable with the original people who wanted it, elusive because the community is alive, and false because no community is infinitely valuable.