Measuring public relations return on investment, or any marketing return on investment is always a dicey matter. Unless your marketing expenditure equates to a direct increase is sales or a favorable upturn in public opinion, it is really guesswork.
Social media has many distinct advantages. ROI measurement in the form of trackbacks, blog posts and comments is immediate and qualifiable. Companies and agencies also get to be a mouse in the proverbial corner during conversations between consumers.
The best reason to be in the social media arena? You consumers expect you to be there. If you're not, they'll be looking for other places to be heard. The social media paradigm is here to stay. You might as well get on board sooner rather than later.