This depends on whether they have a plan, a strategy, or just hang out. There is a lot of shiny stuff online. These would be the games you can play on Facebook. It can also be the links that you follow on Twitter just because you are curious.
There are different opinions on how much time you should spend on each aspect of social media. Anywhere from half your time reading and listening to even more than that.
If you take the time to plan up front, strategize what your objective is and work towards that goal. With automation tools available there are a lot of time saving techniques to get your message out and then allow you to check in when you have the time.
For a larger company, there may be an opportunity to develop one strategy and then share the responsibility among many people to get it accomplished.
What are your favorite time planning methods for handling social media?