Which social media channel does your business get the most engagement on?
We really enjoy Twitter and find it to be very engaging. The key is to put in effort to make tweets a dialogue. Ask questions. Respond to others you follow or that just post interesting and related content. There is nothing wrong with streamlining your posting process, but don't work toward total automation. It's about thoughtful conversation.
LinkedIn groups are also very useful in the same way. You can learn a lot by lurking and just reading the content, but you'll get much more out of participating.
The last network I'll mention is focused more at Entrepreneurs and Startups (however, you can just join as an individual). It's called Teamstory (currently only on iPhone but they are looking to expand). If you are looking for feedback from other entrepreneurs or just looking to connect with others going through the same struggles, it's a great place to checkout.
Lastly, I'm going to double down on saying approach social networks as two-way conversation (not a rant, monologue or diatribe). It is called Social media for a reason, so be social.
Whichever one I am spending most time and attention on. Currently that is Facebook. I have also had great results from Google+. I am in the process of finding the balance and chooseing my main 3 on whihc to focus.