A Distribution, an Audience, and the Passage of Time
Gentoo Linux has had a rough time of it the last few years. James Bellenger has briefly detailed the migration from Gentoo, and some reasons why it is occurring. He wonders where all the Gentoo users are going.
Matt Asay at CNET suspects Ubuntu's rising star is responsible for the exodus rather than Gentoo's lack of strong leadership.
Having used both distributions extensively, and strongly preferring one, I agree with Asay when he says that given Ubuntu's popularity, Gentoo's stats seem far less impressive. That could be said about thriving distributions when compared to Ubuntu.
However, an average Gentoo user is usually not asking the same from his machine as a typical Ubuntu user. That is not to say that all Gentoo users are power-users, or that Ubuntu users never want to get under the hood.