QUOTE(MBisanz @ Sun 22nd March 2009, 9:55pm)

I believe that is a picture of the
Chernobyl disaster and that Jon is saying Wikipedia has already exploded and can't be fixed.
More likely, he means that Wikipedia's inevitable abandonment by its user community won't be accompanied by the removal of the site itself. The site will therefore never really be cleaned up, but instead will remain as a blight on the internet landscape for many years to come.
As for this whole question of "critical mass," IMO it would be wrong to suppose that there's a "magic number" of active editors/admins, below which Wikipedia will cease to be functional or worthwhile. Every person is different, requiring varying levels of community interaction to remain "active," and every topic area is different, requiring varying levels of maintenance. And as Jon seems to be suggesting above, user-base numbers aren't proportional to the usefulness and quality of the information those users "generate."
You'd also have to take varying ratios of constructive vs. destructive activity into account as the community's numbers decline. If the ratio of "good" editors to "bad" editors (and edits) remains constant during such a decline, then you could probably assume that the future Wikipedia would be mostly business-ass-usual
(pun intended), and that a "critical mass" number could even be theorized to some extent. If the ratio decreases significantly, then you have to assume that some degree of lockdown would take place. Whereas, if the ratio increases significantly, then you'd have to assume that most of the administrators would begin to become superfluous and the personal narcissistic motivation for WP community involvement (i.e., power, praise, recognition, territorialism) would begin to evaporate, leaving behind a core group of "true believers" to manage an increasingly drama-free, and therefore dull and tedious, site environment.
Either way, it's probably best not to compare a future Wikipedia, one with a significantly reduced-size community, with some of those non-starter Wikia projects. For a wiki site to flourish you need a core-group of about a dozen fairly committed users, and even now I believe Wikia will set up a subdomain on the request of just one person, assuming a similar wiki doesn't already exist and they think the subject matter is of any interest at all (i.e., the "if you just set up the subdomain, they will come" approach).