There are huge network effects at play with youtube. People discover new content (related videos) as they browse the site.

With Dropbox, it seems, to me, like there are much fewer network effects at play. I don't know what percentage of activity on Dropbox involves sharing between Dropbox customers, but I'd guess its low?

I also think its a big deal that I already sign into my Google account on every computer I use; if my files are just going to be there, with my docs, mail and calendar, I'm probably not going to use another service.