Then don't say it. Is what I'm sure nearly every parent has told their child at some point.
Twitter seems to have changed that a lot. I don't know whether it is because people forget that Twitter is a public forum, or whether they believe that the anonymity of the internet gives them license to be rude or nasty, but there are some people out there who would do well to heed their mother's advice.
I think everyone gets to that point sometimes where they are getting annoyed by someone, and you have a choice to make when it happens. Do you a) shoot your mouth off, or b) take a deep breath, make a cuppa and come back refreshed?
I very much think b) is the better option, and I'm sure many people out there do as well. Especially because you can never be 100% sure of intent over the internet. Only 7% of human interaction comes from the words we use to communicate. The rest comes from body language, tone of voice, facial expressions and other non verbal cues. Emoticons help the internet a little bit, but you have to remember to use them. I'm terrible for forgetting, especially on Twitter where there isn't a quick and easy button to press (I use Plume, not the official Twitter app). And sometimes even when you do use them they aren't interpreted correctly. It gets easier to read Twitter when you've met the person, because then you can hear their voice, and chances are you'll know them well enough to know what they are like.
So. If you're getting wound up by Facebook, Twitter, the internet in general, do yourself a favour and go and make a cup of tea.