The law relating to this is complex and undeveloped, with little modern case law as yet. But it may be helpful to try to make some sense of all this:
In law, you can distinguish between
(a) real, identifyable people. (e.g. The McCanns, Amaral, Bennett etc)
(b) Those who are anaonymous, and hide behind a username (e.g Sweepyface, Biretta, Stephen2500, Wonderfulspan, jean-pierre).
And also between
(c) Combatants (for want of a better term ) - those who actively participate in discussions online on twitter, facebook, blogs, forums etc - e.g. those in (b) and those in category (a) who do engage in online activity. (e.g. Bennett, and those whose real identity is revealed
(d) Non Combatants - those who do not engage in online stuff - e.g. The McCanns, Amaral.
Of course the boundaries between these can get blurred, but for the sake of this they will have to do.
In law, while someone is identifyable only by a usename, they cannot suffer injury. i.e. J-P can call Stephen2500 or wonderfulspam or biretta whatever he wants, and accuse them of whatever he likes, with no consequences at all in law. So getting exercised about Vee's posting is not correct - at this point. There is no criminal or civil action that can be taken.
All users must be aware that they may be identified. Maybe by being "outed" But also their ISP can be forced to reveal details of the user, where there is a possibility of civil or criminal action.
So people in categories (b) and (c) can go at each other hammer and tongs, if they so wish. In fact some forums have a sort of "rumpus room" where anything goes.
But as soon as they start on categories (a) and (d) they run a serious risk of falling foul of the criminal law, or of facing action for defamation.
Of course, once someone is actually unmasked, they may themselves then fall into both category (a) and (c). At this point they may be able to take action against those abusing them, and invoke a criminal complaint or a civil action. However, their own posting history would be considered, and the principle of volenti non fit injuria (to he who is willing no harm can be done) will come into play, and so someone with a vitriolic posting history who is subsequently unmasked and identified would be unlikely to succeed in any legal action.
Which underlines the sense of only posting things online that you would say face to face. You never know when you might be called on it. This applies to both sides, of course.
But I would be fascinated to see evidence of the actual scale of abuse directed at McCann doubters. (those in category (b) and (c) do not count, obviously.
