Some good questions raised here, with particular reference to Peter Singer; an excerpt:

It’s been 30 years since Animal Liberation was published. I don’t know how many times I’ve heard or read about its influence, which is said to be significant. But where’s the evidence for this? Has anyone done a study?…[H]ow much of the book’s influence is due to its argumentation, as opposed to the emotional wallop packed by its images and descriptions of how animals are used in laboratories and on factory farms? If all or most of the book’s influence is due to emotional factors, then neither Singer nor his philosophical colleagues can take pride in the fact that it was written by a philosopher; for another book that had the same images and descriptions—but not the arguments—would have had the same effect.

Philosophers are usually careful in making factual claims, since they have no factual expertise, but for some reason (self-interest?) they take liberties when it comes to making claims about the influence of this or that philosophical work.

UPDATE:  Chris at Mixing Memory points out to me that some of these issues are also discussed in the debate between Peter Singer and Richard Posner here.