Firstly get more people to use the platform and get as much feedback as possible. Once we know where they maybe having issues and what they really like we can know for sure how to make the on-boarding experience truly great. With that feedback, we can use these techniques to make sure they really understand the platform and want to use it again:
A short video under 30 seconds (audio not required) to explain how you can use the platform and get started.
After sign up, a prompt to personalise your interests/topics feed and be encouraged to answer questions in those topics you like/and or ask questions for rewards.
Use "microcopy" to give more info/explain any field that people aren't sure about when they post a new question.