If you are a paid member of the web site, this would count as a discount or rebate and not be taxable.
If you are not a paid member, and this voucher is a reward or payment for providing a service (creating content on their forum), then it is probably income earned from working and should be reported as taxable income. (that is, the value of the voucher in USD based on what you could buy with it).
Barter is taxable income if received in return for goods or services.
