How do you become part of the conversation (on social networks)?

Listen. Always pay attention to what people are talking about your niche market. You can set up alerts for that.
Interact. You are on social networks to build trust around you. So, after you listen to people’s problems interact with them and try to offer them a solution
Listen some more. See if your actions were successful.
Provide quality. Be a constant provider of quality information: either we are talking about content or suggestions (tips, advices)
Be ready to manage a crisis. You have to expect that not everyone would like you. So, if someone talks bad things about you, it would be a good thing to have an already thought plan on how to react.
Be the starter. Sometimes you should be the one that starts a conversation.
Don’t talk bad things about someone: eventually it will turn against you
Be sincere. This is valid on all social networks. If you are planning to post direct sales make it clear in your profile. Otherwise people will start to ignore you.
Never spam.
If you are entering a conversation don’t do it just like that: add something to it.