The final factor that is of influence on critical mass in collaborative web-applications, is that of rewards. Publishing in academic journals is tightly integrated with career advancement, and thus brings great rewards to authors. Compared to this, web-communities can offer much less. Though it is likely that this will (or at least can) change, as IT-people already do receive career-benefits from web-communities.
Other forms of rewards are rankings and reputation-points. These can signal the reputation of members, and thereby motivate excellent community-members. In addition, as Gerard Beenen has demonstrated, things such as simply assigning people to teams (even if just in name), and inducing competition between these teams, can make people more productive community members.
Another important way to make virtual rewards more rewarding is making them recognizable. So instead of a karma number that goes up from zero to a hundred, creating a small set of classes of expertise, mimicking those in society (such as layman, student, postgraduate, etc...), is more effective. Making reward-points artificially scarce, or allowing users to re-invest them (to feature their own stories), is another way to make them more valuable. For example in IBMs Beehive they limited 50 (randomly selected) users to rating only three items each week, and this resulted in more, rather than less, ratings.