One thing that probably makes your book marking tool useful is the fact you charge per use, not free, and not monthly. There are many tools out there where you buy them for X and then use them a couple of times (like a normal person...), and then there are others who are blasting out utter crap millions of times a day. The people who own those blasted sites need to do something, so they block the tools, blocking your stuff and blocking well thought out bookmarks/articles/etc. Putting a small price on everything keeps people from just going wild. A monthly fee encourages as much abuse as possible, while a per use fee usually keeps the quality up.
I like the idea of tools that creating other decent backlink strategies. Even decent forum profile creation would be good, posting to forums, etc. But in such a way that you're not encouraging mass abuse and/or utter sheer spam spreading.
Producing a product that creates quality backlink for the user but also encourages quality back linking strategies from the user.