On Monday, footballer Ryan Babel was charged with improper conduct by the Football Association. In a reaction to Liverpool's exit from the FA Cup, he posted to twitter a picture of Howard Webb - 'photoshopped' to show the referee in Manchester United kit.

That is as much of the incident as has sometimes been reported, perhaps because the image of a referee in a team kit grabs attention, and I'm sure also because it's funny. In fact, mightn't this have been just a joke? Isn't this reminiscent of (though not quite so horrifying as) Paul Chambers' recent conviction?

Actually, in this case, there was a little less ambiguity than the picture on its own would have preserved. Babel's tweet included the line, "And they call him one of the best referees? That's a joke. SMH [shaking my head]." 'Best' is indeed a subjective opinion when it comes to referees, but the fact remains: he is the referee, and Babel's tweet was a sincere criticism.

For me, I think this puts Babel's indiscretion in the same category as Pete Broadbent's disparaging remarks about the royal wedding, posted on Facebook. We're all entitled to make a frustrated and flippant joke about a service provider, even it's a little distasteful; and even MPs, like Gareth Compton, can be allowed to make distasteful jokes; but a Church of England bishop can't really get away with a sincere public attack on the monarchy any more than a professional footballer can get away with one on the referee.

It's tough to judge: hardly the clearest cut of lines. Perhaps you could say, the difficulty is in properly sensing responsibilities (and their transgression) in the context of an unfamiliar medium. There are two important things with which I think it's worth getting familiar:

Even if it's a 'social network', it might still be a public medium. That means journalists. Children. Security professionals. Your boss.

The written communication of irony will often fail, especially with only 140 characters; and equally it's difficult to ascertain - once in the court room or equlivalent - that it should have been perceived.

I loved Charlie Brooker's withering response to the judgement on Chambers, and it would be great to be able to keep a new medium like twitter only for perfectly grown-up laid-back people who are hard to offend and who in any case enjoy a good dark joke. The problem is, when a new medium is successful - and really worth caring about - that's probably because it's accessible to everyone, including Disgruntled of Tunbridge Wells.