Both the developers of these systems as well as researchers working on the subject matter frequently claim their approaches to be searching the entire web or, at least, to be scalable to web size.
there is hardly any evidence to substantiate this claim-rather the opposite can be observed: commercial plagiarism detectors have not been found to reliably identify plagiarism from the web (Köhler and Weber-Wulff, 2010), and the evaluation of research prototypes even under laboratory conditions shows that there is still a long way to go (Potthast et al., 2010b).