Sites don't respond well to stuff that misleads users into thinking that they're using an offical product, and will take it down without regard for any legal arguments. I don't have a trademark for Pinboard, but getting stuff that uses my logo or site name removed has been dead easy the few times I've needed to do it.
You might not have a Registered Trademark, but if you use a name or logo in commerce, you still have a common law trademark and still have protection against others in the industry using a name or logo that is confusingly similar.
