There are many reasons why social media is important to effective PR, and here are a few.
1.) There are one billion people on Facebook. There are 500 million users on Twitter. Social media is where the audiences are.
2.) Social media is short and to the point, most posts are 140 characters or less. Short blasts of information are important because in the world today people are busy, no one has time or attention span to sit down and read long articles or press releases. People want things that are quick and condensed.
3.) Audiences have opinions and want those opinions to be heard. Social media allows customers to voice their (usually wrong, abusive and evil) voices.
4.) Akin to the point above of giving your audience a voice, it also gives you a voice to talk to your audience. Social media allows you to address issues and problems to those it affects directly.