A good rule of thumb: The more mainstream/top-of-mind a service is, the more novice users it’ll attract.
If a visitor uses a fringe service, they’re more likely to already have bookmarklets/toolbars/applications installed to use that service. So, focus on the folks who don’t know how to “trick out” their browsers.
A good example is Delicious (still the brand-leader for social bookmarks) vs. Pinboard.in, which positions itself as a sort of anti-Delicious by having “get acquired by Yahoo and slowly grow useless” as a future roadmap item.
