I used to personally search all of my students papers without the use of some other site... I decided for myself if it constituted plagiarism or not. Some of the accusations are ridiculous, particularly if it's a simple sentence stating a fact (or educated opinion) that can only be stated a certain number of ways. For example, the sentence: "Shakespeare is the most famous author of all times." I typed that here.... then I searched the web, and WOW, it found an exact match. Did I plagiarize it from that site? It's exactly word for word.
Now, if I were researching authors, and happened to use that site as a source, and based on what I read there, I rewrote it to say "the most famous author of all times is Shakespeare," THEN, if I didn't cite the source, it WOULD be plagiarism. (for those of you in high school who don't have a complete understanding of plagiarism, EVEN IF you rewrite the sentence in your own words, as long as you gathered that knowledge from another source - it's plagiarism if you don't site that source.
As a result of the overwhelming majority of papers that I've ever received being extensively plagiarized, I no longer assign papers. (I used to offer them as extra credit projects... only 1 person ever got the credit, every other paper was plagiarized, and it took me too much work to prove it for each paper.)