Agree with Kyro. Try to go as far as you can without any marketing budget. It takes a lot more effort, working on getting discovered by users, but I don't think it's any less effective than paid marketing, and possibly even more effective in the initial stages.
Great concept! Certainly does help to structure tags that can grow out of control in any tag cloud. I'm wondering if you've considered building this as an API or dev library so that it can be used by developers not just for bookmarking but also for tagging in general. But then, where's the money in that, eh? (^_^)