As defined by Michael Hauben, author of the book, Netizens:
"...people who care about Usenet
and the bigger Net
and work towards building the cooperative and collective nature which benefits the larger world."
"...people who work towards developing the Net
You are a Netizen if you contribute to the online community in a positive fashion, allowing the community to grow.
Tom is the epitomy of a Netizen, he spends all his free time in those tech support forums answering people's questions about computers.