The definition of critical mass may vary widely but the concept remains the same. It is a quantity of something that above which will cause a chain reaction.
How does this tie in with websites? What drives websites first and foremost are its visitors. When we think of the critical mass of a website we are thinking about how many active visitors the website needs to have a chain reaction. Chain reaction in this sense will describe drawing the attention of more visitors to the site.
The attention of additional visitors to a site may only be gained by via promotion of the site. This promotion may be done using advertisements (which sometimes costs a lot) or by using the users themselves. We will focus on the form of site promotion that uses the existing users themselves to promote the website.
There are 2 main types of websites:
We will look at the characteristics of each below.
For the most part, social networks have a much lower critical mass but the "chain reaction" also dies off faster. Social networks have a lower critical mass since the sole idea of the network is to be social in one way or another. Inherent in this system is the idea that users will almost automatically and mindlessly invite their friends to be a part of the network. This improves the number of visitors that a site will get relatively quickly since each user will invite their friends and their friends will invite more friends and so on.
All this is fine and dandy for social networks except for the fact that the friends of a user need not be interested in what the particular social network has to offer. Therefore, even though it may be quick to attract a number of users faster, most users usually go away or get bored after a while. This is when the website reaches "full maturity", or when there are no more users who are interested in that sort of topic.
At full maturity, the number of new users falls sharply and continues to trickle until the death of the website and a new social network takes over.
Content driven websites on the other hand have a true critical mass. The number of active users required is usually much higher than that of a social network, but once that mass is reached, the site will continue to attract more users until it reaches its "full maturity".
The main difference with content driven networks is the way new users are attracted to the website in the first place. Search engines pull the majority of the traffic to a content driven website. What determines the ranking of a website on a search engine result page (SERP) the most, is the number of incoming links to the site. Only when a site ranks high on a SERP will it get substantial traffic. That is, the best thing to promote a content driven website is incoming links.
This is why content driven websites have a much higher critical mass. For every quantity of traffic that it receives, only a small portion of those visitors have the facilities (a website or blog for example) to link to that website. The payoff is slow but great, because once the incoming links add up, the site will continue to rank higher and higher on SERPs thus attracting more and more visitors. This results in a true chain reaction.
What we can see then is that social networks mature faster but also die faster. Content driven websites are the true champions as they will always continue to get more traffic as their age, reputation, and number of users (who generate the inlinks) go up.