Unless US law is different then I'm aware of (and a quick bit of research suggests it is not) defamation (liable or slander) lawsuits require saying/writing something that is false. Here's the OKCupid statement:
"Hello there, Mozilla Firefox user. Pardon this interruption of your OkCupid experience.
Mozilla's new CEO, Brendan Eich, is an opponent of equal rights for gay couples. We would therefore prefer that our users not use Mozilla software to access OkCupid.
Politics is normally not the business of a website, and we all know there's a lot more wrong with the world than misguided CEOs. So you might wonder why we're asserting ourselves today. This is why: we've devoted the last ten years to bringing peopleâ"all peopleâ"together. If individuals like Mr. Eich had their way, then roughly 8% of the relationships we've worked so hard to bring about would be illegal. Equality for gay relationships is personally important to many of us here at OkCupid. But it's professionally important to the entire company. OkCupid is for creating love. Those who seek to deny love and instead enforce misery, shame, and frustration are our enemies, and we wish them nothing but failure.
If you want to keep using Firefox, the link at the bottom will take you through to the site.
However, we urge you to consider different software for accessing OkCupid:"
It seems to me that the statement consists of statements that in for far as the public record is concerned, are true. E.g. "Brendan Eich, is an opponent of equal rights for gay couples.", which is supported by the contribution that started this all; the rest of it appears to be statements that either relate to feelings of OK Cupid, or clearly deliminated opinions. IANAL, but I do spend a lot of time talking to them professionally, and I think it would actually be a very weak case for liable (which is what this would be, slander refers to the spoken word, liable to the written one).
You are welcome to opinions on how OKCupid handled this, but I think the argument that it's legally actionable is probably incorrect.
In brief, in order to be defamatory, a statement must be:
1) Public (e.g. someone had to have heard it other then the two parties)
3) Not an opinion
(there's a couple of other items that have no baring in this case)
I think anyone reasonable could agree on 1 and 4, but 2 & 3 have larger hurdles.