If you’re not going to talk about me to your friends, I don’t want to know you.

That’s the message that your web site is sending if you only accept registrations from Facebook and Twitter users. You can see what I’m talking about at Pinterest, a social network for image sharing that only lets you register if you use Facebook Connect or Twitter sign-in as part of your sign-up process.

I’ve been besotted with Pinterest for about four months now, telling colleagues, clients and friends that they must sign up for this handy way of collecting their favorite online images, lusted-after tech products, or fashion and design wishlists. Pinterest makes it easy to bookmark visual resources and organize them by category, and to share your discoveries with friends. So far I’ve used Pinterest in a whole bunch of ways, including to:

• Bookmark clothes I’d love to wear
• Track the sexiest gadgets I spot online
• Compile options for a new computer bag
• Identify finishing options for our upstairs deck so we can agree on a design plan with our neighbour, and of course
• Consult my friends on which grey boots I should buy

One of the great things about Pinterest is that it lets me visually eavesdrop on the style finds of my friends, which is why I was so glad when Tara Robertson and Lauren Bacon both signed up. So there might be a tiny element of self-interest behind the eagerness with which I’ve recommended Pinterest to my most fashionable friends — the same friends I turn to for everything from decorating advice to shoe recommendations.

But not coincidentally, two of my most aesthetic, dearest pals are also folks who stay away from social media. Maybe it’s precisely because they are visual people that they feel less interested in expanding their repertoire of text-based communication; more likely, it’s that the time they spend choosing the perfect rug or the perfect bathtub or the perfect blazer really cuts into the time they have free to tweet.

Take my friend Tamara, who I visited last week. On my last night in town we came home to her house, which I would describe as halfway through a bathroom renovation except that implies that the bathroom will one day be finished.  That day, the contractor almost finished laying the marble floor, but when Tamara looked at the way it had turned out she didn’t like the variation between the white with a little grey of some tiles, and the grey with a little white of the other tiles. At midnight I got to smash marble tiles with a hammer and pry them up with putty knife. Fun! (Really. You should try it.)

I suggested to Tamara that using Pinterest would be a good way for her to compile her design choices for the remainder of this project so that she could keep track of which products she wants to order and where she found them. She’s usually not a big one for signing onto my latest social media enthusiasm, but Pinterest really spoke to her and she jumped on the invitation I sent her. But no sooner had she clicked the invitation link than she was taken to a page that asked her to link her Pinterest account to Facebook or Twitter in order to finish registration. Since she’s not on Facebook or Twitter, that brought the sign-up process to a halt.

Why would Pinterest limit sign-ups to people who have a Facebook or Twitter account? In this thread on Apartment Therapy, a Pinterest rep says it’s to ensure that everyone on Pinterest has a real identity. And if Facebook or Twitter were core to the Pinterest experience, I might say that makes sense.

But I suspect another dynamic is at work. For a pre-launch beta like Pinterest, in which the object of the game is to build buzz over an extended period of time, people like Tamara are dramatically under-valued. After all, as a non-tweeter and non-Facebooker, Tamara won’t do a lot to increase the Pinterest buzz. And if she’s annoyed or turned off by their de facto gatekeeping, who cares? It’s not like she’s going to tweet or post about it.

The urge to focus on “influential” adopters makes sense for many social media launches — certainly, any service that is going to primarily appeal to people who are already active Facebookers or Twitterers. For example, it’s hard for me to imagine too many people jumping on board a FourSquare or Gowalla without a previous life on a social network.

Pinterest is a different story, however: its functionality makes it specifically interesting to the kinds of people who may have resisted other kinds of social networks. Folks who connect with images more than words; who pursue home or craft projects rather than blogs; who are more interested in tracking their own finds than in sharing with others: these are exactly the people who might fall in love with Pinterest. And yet Pinterest is shutting them out by stating that only the socially networked need apply.

If Pinterest has made a strategic error — and lost out on potential audience — it’s not alone. Every company or organization website that relies on Facebook Connect or Twitter OAuth as its exclusive login system, or that creates its social media presence only on a preexisting social network, is potentially making the same mistake. True, you’re making it easier for people to find or join your social media presence if you go to where they already are. But unless you’ve got very clear reasons for believing that 99.9% of your audience is already on Facebook or Twitter, you need to leave other ways into your presence, too.