I looked at the Common Knowledge page for a few books when the feature was first featured on the LibraryThing blog. I think it must have been when it was originally introduced. I thought it was kind of fun and wanted to find a way to contribute but I don't think I ever did.
I do find it interesting (just like movie triva!) and need to remember to check it out more often. I think my only concern is that a lot of times for the books I'm reading I might not find anything. I think for classics there will be a lot of Common Knowledge but for newer books maybe not so much.