While it depends on your audience as to how much effort you put into your social media marketing and outreach, I think every company or freelance professional should at least cover the basic Facebook, Twitter, LinkedIn websites.
If they can find a way to make the media apply, certainly go into YouTube and any or all of the picture-based social media websites (Picasa, Tumblr, Flickr, etc.). Some industries also have industry-specific social websites and you can also utilize MeetUp, etc. And don't forget about blogs!
Most of my past marketing positions and internships were based in social media outreach, and there are unlimited ways to connect to your audience. The trick is making sure you hit each individual from multiple platforms.