Do majorities determine facts? How about motivated minorities? Do they determine fact? This is an issue that the popular Wikipedia has had to deal with.
CNET has an op-ed piece about Wikipedia and how its growing popularity -- and growing use as the defacto standard on "what is true"(TM) has highlighted the fact that it's open to abuse, disinformation, and plain old error in ways that "closed" knowledge bases are not. That's not to say that encyclopedias don't make mistakes -- they certainly do. Moreover, traditional encyclopedias and other closed mediums often have hidden agendas that the reader may not be aware of.
However, Wikipedia's openness means that anyone can modify any page regardless of merit. For example, this Fall, a disgrunted WinCustomize user used the WinCustomize Wiki entry to trash moderators and admins they didn't like. Even now, over half the entry is dedicated to "criticism" of WinCustomize due to forum arguments (where according to logs, less than 1.5% of WC users go).
Read CNET's excellent commentary on the subject.