It's something that we've all struggled to initiate. Part of this is why the bounty program includes site advocates. They start the conversation and then people join in. Q&A's are generally a good way. Hold a Reddit style ask me anything around big news to help kick start the conversation. You may need your advocates to get a couple of questions pre-loaded so your Q&A turns into something.
Turn it into a video where you mention the person's user name in the video... people live the attention.
Opinion pieces are going to generate more responses as people either agree or disagree with your take. The news is the news, but your opinion is open for debate.
Direct message people who are interacting with you on facebook and twitter.. ask them to come participate on your site. You want them there, because Twitter has too many trolls... personalize it for them.
just a few ideas.