It is pretty much unheard of for peer-reviewers to be paid for their work, and PLoS certainly doesn't do it. In the case of an open-access journal such as PLoS ONE, reviewers are contributing to the world's body of science. In the case of paywalled journals owned by the likes of Elsevier and Springer, they are contributing to shareholder profits. That's why I wrote this piece on the Times Higher Education advocating that we refuse to peer-review for non-open journals: http://www.timeshighereducation.co.uk/story.asp?storycode=41...