I don't see much of a dip in time on site. Note also that Alexa is heavily biased (who exactly installs that toolbar?) and that time on site in Alexa is a blend of time on site for new and old style documents. I know nothing about scribd's metrics besides what I read on the internet, but my guess is they're discussing time on site for exclusively converted docs, which you will not be able to see on Alexa.