Since Jottit doesn't link to its users' pages, their rank in the beginning is 0, and so is their attractiveness for spammers. Once Jottit builds a catalogue of user pages, it's dead.
If they build a list of links to users' pages somewhere on the main site, which presumably has a solid PageRank (I'm sure it will after some time), that would automatically boost users' PRs as well, no matter what's on the pages. And so the spam flood will begin.
