As I've been saying for a while, women's massive underrepresentation on corporate boards is a huge tell about the state of gender equity in the country and the world. There are no real rules or guidelines as to what constitutes a well-qualified candidate for a non-executive director role, and frankly most companies don't want active boards that really supervise the managers. If you care even a little about gender equity, you'll find some women to put on your board.
Alternatively, you could find yourself in the situation Twitter's in where you say you're eager to add a woman but haven't yet quite found the right person.
The traditional thing to do with boards is to give the seats to retired politicians or to executives at other companies. And since most retired politicians and most big time corporate executives are men, that kind of practice makes it "hard" to find "qualified" candidates. Obviously, though, there aren't literally zero women who fit these descriptions. I'm sure if you locked everyone in a room for two hours and made them come up with a list of women who fit those descriptions they could get it done. But why not think outside the box? How about someone like Katy Perry, Lady Gaga, Taylor Swift, or Rihanna all of whom have upwards of 30 million followers? Surely their success in the medium could give the company some important insights about its use as a platform by celebrities. Or in light of the teen market's importance to Twitter, how about Tavi Gevinson? And of course you don't need to limit yourself to just one woman. You could have a celebrity twitter user and a teen media entrepreneur and Ellen Tauscher (or whomever) on your board if you wanted to.
In fact, the genius of board of directors gigs is you can do anything you want. To have one or three or seven women on your board, you just have to genuinely desire to have gender diversity on your board.
UPDATE: Another good somewhat outside the box suggestion is Shonda Rimes, creator of Scandal the most-tweeted-about show on television. Given Twitter's interest in "second screen" content strategies that would be a natural area for the company to add high-level expertise.