You need to know why your members are participating in your online community.
They're participating because it benefits them, but you need to be more specific than that.
You need to know if it's because it makes them happy (control, be effective, learning) or for rewards (money, power, fame, belonging) or out of fear (stay employed, ego-involvement, crime…).
Ideally, new members should be asked whilst registering why they want to join your online community. But if it's it’s too late for that, you can ask 5 members a day by e-mail. Or you can ask the entire community at once with a blog post (quicker, but you'll get a worse response rate).
Identify similar motivations. You’re not going to get the same answer from everyone. Once you know why your members are participating, remind them every few months. Be specific about what members have learnt recently, or how much more well known they are now and how they've reduced their likelihood of being made redundant.
Your community should always help every member get where they want to be.