Understand your client’s goals and how they would measure success on those goals. Then, I’d suggest you do some research for both research/studies and individual case studies that are pertinent to your client. Examples for some typical goals.
Increasing sales via word-of-mouth: see the data related to the influence of positive reviews from social sources: http://www.bazaarvoice.com/resources/stats
Increasing brand awareness–See this recent study about social media impact in Millennials http://bit.ly/etTmC7 and this one: http://www.deweekkrant.nl/artikel/2010/november/23/mandarijnen (note that this shows that social media work can increase click-throughs on your PPC paid ads)
Increasing website traffic–Some data re:blog benefits http://bit.ly/g4NCLA and this report from marketers: http://sports.yahoo.com/nfl/blog/shutdown_corner/post/Win-and-they-re-in-The-Late-Games?urn=nfl-302545&utm_source=twitter.com&utm_medium=referral&utm_campaign=br_seahawks
And here’s a nice list of some diverse case studies, though I would suggest finding one’s specific to your client’s circumstances: http://techcrunch.com/2010/07/17/how-social-media-drives-new-business-six-case-studies/
Note also this McKinsey study which just came out where businesses report benefits from different “Web 2.0″ efforts: http://bit.ly/gAycd6
I find it useful to do a couple of things in a casual presentation:
1. Start with a business case for a PR company. You and your client will find a lot of similarities.
2. Show the good case studies where companies have said “We made $X million from this” for example Dell on Twitter, Moo.com on Flickr.com etc.
3. Show how by not engaging you are still going to be impacted financially (Think of what happened with Nestle, Northwest, and American Airlines).
4. Use metaphors when explaining it to people who don’t really get it. I’ve often explained it like this. Imagine you’re at a dinner and seated at the table are a group of potential customers. Also at the table is someone who is bad mouthing your company in front of these potential customers. Are you passive or do you attempt to address the comments?
5. This advice depends on the clients business model. If they offer technical support of any kind, there is also a business case around the money they’ll save by using social media to provide support to clients – way cheaper then a phone call. If they spend any money on marketing, there is also a case for the money they’ll save marketing online which is person-for-person about the cheapest marketing you can do.