I created a function using WordPress for rewards system and referrals.
What happen in my site .
If any user login he/she will get some points and for each type of post like question/answer/comment/blog post they will get some points and using those points they can get some gift vouchers.
Check the my site here For Q/A --> https://haveqa.com
For Blog post --> https://haveqa.com/wp
For Rewards points--> https://haveqa.com/rewards