All forms of social media have good and bad points. The fact of the matter is that like it or not, social media is here to stay.
Anyone who abuses someone else online is subject to the same laws on defamation as a newspaper journalist would be.
I'm just going to slip in to law student mode for a moment whilst I explain. Article 8 of the European Convention on Human Rights (ECHR) gives the right to respect for private and family life, meaning that people's private business has the right to remain private, so if you publish on Twitter a comment about them that is not in the public's interest to know, especially if it turns out to be untrue, you could find yourself with a defamation case on your hands.
This is balanced out by Article 10 of the ECHR, which gives the right to freedom of expression, so if you believe that the public has a right to know about something, then you have a right to publish it, which of course also covers publication on a social media site.
The courts try to take in to account both Article 8 and Article 10 when deciding on a defamation case, as neither Article has a natural precedent over the other, but be warned that if they decide that the right to respect for private and family life over rides the right to freedom of expression in a particular case, you could find yourself in deep doo.
You have been warned! 
