And speaking of trying social networking/media sites, people frequently want to know, "Where do I start?" Great question.
Don Reisinger on CNET does an assessment of critical social categories, what works for him, and settles on the same set of tools that I would recommend.
Delicious for social bookmarking.
Twitter for micro-blogging.
Reddit for news-aggregator.
Facebook for social networking.
YouTube for video.
I would also add Flickr for photo sharing due to ease of use--they even make it easy to retain rights to your photos--and sheer volume.
Not all of these will make sense for your agency social media strategy, but like I said before, you need to get your own feet wet.Read the complete CNET article here.