It's a service built on top of Pinterest, using a confusingly similar log (same color and scripted P), which uses the word "pin" to refer to a piece of content posted on Pinterest.
Throw in some confusion of trademarks with "you can't copyright a font" for good measure and you have the perfect regularly scheduled "I'm being screwed by a big company that I won't bother to mention is being totally reasonable" post.
While IANAL, the issue isn't just "pin" — it's the word + the color + the font, all of which make the logo a lot more similar to Pinterest than the word alone suggests.
Or maybe they were thinking more along the lines of honey versus vinegar, if we're going to try reducing something as complicated as trademark law to clichés and homilies.
When people advertise your brand for free, even in the smallest way, you are behaving rather foolishly when you start sending threats to them.
This is a relatively minor incident in the grand scheme of things, but Pinterest is just shooting themselves in the foot here.
While there are some complicated legal issues involved with so-called "defending your IP so you don't lose it", the biggest thing I see here as a developer is that one of the bigger social networks is using its time and energy to crush somebody that is effectively advertising their brand for free and is quite literally no threat to their interests.
As a developer, I see what companies like Twitter do when they get popular and make it difficult for others to build products and this makes me concerned. When Pinterest inevitably releases their own API, why should developers invest time and energy developing products that benefit the Pinterest brand when there's a chance that the effort will be wasted because their lawyers are trigger happy?
Pinterest is one of a handful of companies that has a chance to sort of challenge Facebook down the road. My advice is that their management should work with their lawyers to avoid screwing their own reputation. This is salvageable for Pinterest and an opportunity for them to clarify that they are developer friendly and not a dumb, faceless corporation like Twitter has become.
You also can lose your trademark if you aren't shown to be actively defending it. This is why Google, for instance, resists efforts to turn their name into a common verb.
IANAL, but I think trademark violations don't hinge on just one element (i.e. name, or in this case, prefix). It's the combination of the name, logo, and the use of a pinterest.com URL as the central piece of this service, which might confuse users, thinking this service is affiliated with Pinterest. I think their letter explains this pretty well, and I couldn't spot the "threat" in it? There was just a vague hint of "consequences".
BTW, http://pinteresf.org now redirects to pinterest.com. I wonder if they got a similar letter, and as a result handed over their domain?
Though these can probably be used in court proceedings as "defendant previously received notice/requests from us... which were not complied with" or something along those lines
It looks like most of those have been applied for but are not yet registered. I'd be very surprised and (shocked/dismayed/appalled) if Pinterest was able to register a trademark for the word "PIN".
That said, INAL (I'm not a lawyer), but I believe legally they could argue that because you are using a word that is Prefixed with "pin" and that your product also a service related to Pinterest that there could be confusion as to whether PinPigeon is a product of Pinterest's or not.
TINLA, but to be honest, I don't have much sympathy for the original author. The letter wasn't threatening at all, it was polite and worded nicely. Its contents also pertained to the use of a Pinterest-styled logo, the similar typeface, and the general look and feel that made the site feel a lot like an official Pinterest-authorized site.
I thought the same as soon as I visited the site as well.
Throw in some confusion of trademarks with "you can't copyright a font" for good measure and you have the perfect regularly scheduled "I'm being screwed by a big company that I won't bother to mention is being totally reasonable" post.