I don't think what Facebook did was unethical. It's certainly legal. They catch a lot of flack over privacy issues and they want to remind the media about their competitors' privacy issues and influence them to cover those more, so that they don't look as bad in comparison. That sort of work is done by PR firms, so they hired one to do it. In fact, they may not have even asked the PR firm to do it; the PR firm may have acted independently as part of their service.

As for the PR firm's offering to help write an article... Well, I would call it bad/unethical journalism if such an article were published as reporting. If it were published on some opinion page or like a tech blog, that would be fine.