Facebook's experiments bother me more than OKCupid's. They're deliberately manipulating which news stories their readers see in order to affect their mood, and seeing how that affects the readers' behavior. That seems mean and dishonest. (Of course, I didn't know Facebook had news, so I'm not in their target market anyway, but it still seems mean.)
I agree that the Facebook incident is much more worrying than what OKCupid is doing.
OKCupid's a dating site, which means that all their "compatibility" scores are pretty much guesswork anyway, assisted by a lot of measurement, so an occasional suggestion of "maybe you two should see if you want to date" to people they normally wouldn't match up isn't that much perturbation of their approach anyway, and "whoops, pictures are broken, why don't you try talking first instead of just looking at pictures" is just fine, and both of them give them a bit of data outside the ranges they'd normally be collecting from - perhaps there are people that would get along well who they haven't been matching up. (I'm not in their market either, fortunately.)
Your post raises another interesting (and IMO ridiculous) issue though, which is that just because providing either of two different services (either pictures or no pictures) to all of your clients is perfectly fine - randomizing people to receiving one or the other is often considered not to be ethically sound.