Spread adoption
Once one group is using the wiki as a core collaboration tool, you now have Champions who can help you spread the word to more groups and departments.
More people using it means they can each encourage their direct contacts and collaborators to join, thus laying the groundwork for exponential growth. This Viral growth increases the value of the wiki to each user, because the number of people with which one can collaborate increases.
These wiki champions are very valuable to spreading the wiki because of their existing relationships with colleagues in other groups, people it would take you much longer to seek out on your own. They can introduce these colleagues from other groups to the wiki by using it for projects involving multiple groups. The new wiki users will then encourage their own groups to adopt the wiki for day to day collaboration.
⇒ Next: Support emergent behavior