Social media has a lot of potential. The trick is to not think of it as "social media". It is quite simple another networking channel. Most any producer has his/her own techniques (I am assuming!) to find prospects, establish and build a connection with those prospects, and then to close the sale and maintain that relationship. Social media is simply another medium in which producers - if he/she can grasp the appropriate nuances and parallels of traditional networking, and how to do the same in the chosen platform - can realize some great results (sometimes with less time and effort) in prospecting. It is by no means a panacea or a wonder solution that will double sales (for example), but when used effectively it can greatly increase your influence.
Additionally, it can be used as a very cost-effective way to increase brand awareness and agency exposure. Just as you would pay for a print ad, a radio spot or a direct mail campaign, you can get your (albeit brief) message out in front of the people whom you want to have exposure to your brand. This is not automatic and there will definitely be a lot of trial and effort involved, but the resulting cost, and return on investment, can be superior to sustained marketing efforts in traditional channels.
Of course none of this will work if your clients or prospects are not there to begin with or do not actively engage with you on social media channels. There are easy ways to check for that too before you start any efforts.
I do also have to throw out there that the ACT website has some great resources on this topic, including templates for agency social media policies and strategies.