Houellebecq never cites Wikipedia when he copies from it, so could the encyclopedia sue him for plagiarism?
Probably not. Wikipedia is not the author. Each article is a collective work, with each contribution signed with the pseudonym or IP address of the contributor in the History tab. For Michel Houellebecq to get into legal trouble, one of the contributors would need to feel particularly wronged, which is unlikely to happen. Not to mention that Wikipedia sometimes borrows from other texts.
Ironically, a smart-ass contributor found it funny to rewrite Wikipedia's definition of the housefly with the slight alterations Houellebecq added in his book. Houellebecq is not (yet) a member of the French Academy, but he has earned a spot as a Wikipedia contributor. He must like that a lot.