People said that about search in the late 90s. Why Delicious didn't get into advertising is beyond me. With the way everything's tag-based, you'd have had crazy targeting.
The fact that the majority of users are naturally dependent on searching means there's (obviously) massive exposure to eyes and wallets, and there always has been. Whether that exposure could be turned into profits was what was questioned in the 90s.
Bookmarking is different. I'm going to offend some people here by whipping out my anecdotal evidence: I don't know _any_ non-technical (read: average) users who are interested in maintaining a bookmark library, even if the benefits were explained to them.
On the slim chance that we could somehow convince them that it was worthwhile, I still can't see most of them being interested in browsing tag clouds and bookmark trees of other users, which they would have to do in order to be exposed by this 'ultra-specific' advertising. Anything beyond using it as a regular browser bookmark menu that syncs across computers is pretty unlikely in my opinion.
Why not just check Facebook to see what people are looking at? Or, for the 'geekier' of the mainstream, just check Twitter.
xMarks, a bookmarking service with 2million+ users, in their going away blog post alluded to this same barrier:
> We built it and it put it front of potential advertisers. Many were interested, but ultimately the feedback was negative: our user base was too small to be worth their time and attention.