Apparently, everywhere.
I'm being a little sarcastic, a little tongue-in-cheek. Yes, I get pissed off when some trash-talker is dropping the f-bomb every two seconds behind me in the checkout line at Target. Someone swears on their blog, podcast or via twitter? I'm not so bothered.
But if I see a blog entry with the F-word or S-word every few words, I'm thinking this is a classless a-hole.
There's no hard-and-fast twitiquette rule for how much you can or can't swear, just as long as you do it in less than 140 characters.
Facebook, on the other hand, will make you enter a Captcha code if you're a little too judicious with your swearing.
So be mindful. When are cuss words allowed? Not in public, but sometimes in private (or online) for emphasis.