It's very easy, but doing so degrades the value of the site for honest users.

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.